Do you really want a reason.? Will you actually consider the reason? Here are a few for you:
- Culture
- Genetics
- And the biggest one of all: Rules. The game runs on rules.
Here is a question for you: Why can't my rogue gain a totem spirit from the barbarian class? Why can't my wizard learn to have extra skills like the rogue or bard? Why can't my paladin learn all druid spells? Why can't my fighter learn to cast high level wizard spells? Why can't my sorcerer know how to use martial weapons?
There's a huge difference between altering a major chunk of a class and reassigning a single stat.
Example: Upcoming character of mine is a wizard. Due to background issues (among other things, he's a woodworker), I asked the DM if I could substitute proficiency in the quarterstaff (in which wizards are proficient) for the handaxe (in which they are not). They do the same damage and none of the other weapons available for wizards made sense for him. Also, he's like two feet tall. The DM agreed. I have an axe. He can't use it well at all because he's neither strong nor particularly dexterous (as I didn't need him to be either of those things), but he has it.
What this means is that I switched around two things of equal value to better fit my character concept. It's basically identical to switching around a stat bonus to a different stat. What I did
not do is ask for a large addition to my class, such as saying my character's a single-class wizard, so can he also know druid spells. If I really wanted to learn druidic magic, I'd talk to my DM about how long and how much money it would take to research a wizard equivalent of a druidic spell. I could create
Magyar's chained lightning or
Magyar's tree stride. That used to be a thing in 2e, at least (dunno about 3x and 4e), so it could be recreated for 5e.
Now, if you were one of my players and you had a
really good idea for a rogue with a totem, or a druidic paladin or a martial sorcerer, and I thought the idea had merit and wasn't just you whining because you wanted something cool, I'd actually try to work on adjusting an archetype for you!
For instance, for a martial sorcerer, I might let you spend sorcery points and/or spell slots to create a weapon that lasted for a minute. A couple of points or a low-level slot means an ordinary weapon; a lot or points or a high-level slot might let you create a magical weapon. Or, for a more mundane answer, I might just let you have proficiency in
a martial weapon, but take away proficiency in other weapons (between rapier fencing classes and honing your magical skills, you had no time to figure out the crossbow, staff, or dagger), and anyway, you also don't have proficiency in any armor and you have d6 hp so good luck in melee. Plus, multiclassing is always allowed. All you need is one, maybe two levels in fighter and you get a nice fighting style and all the weapon proficiencies you can eat. And Second Wind and Action Surge as well!
A paladin druid? Hmm, let me introduce you to the Oath of the Ancestors. Oh, and try being a human (or other race) but represent your oneness with nature by actually using eladrin stats instead--use eladrin rules, but not lore, to represent your character.
A totem rogue? Yeah, you
stole a totem spirit. Now you have a spirit attached to you that will give a power of some sort but at the same time it eats some of your life energy (you weren't the one the spirit was supposed to bond with, but it's too late now!), meaning you have to spend hit points or Hit Dice to activate a power. Oh, and it'll be painful as well. I'm pretty sure my players would RP the hell out of that. Some of 'em are such angst-monkeys; they love this sort of stuff.
A wizard with lots of skills? Let's see... you can have extra skills, but your Arcane Recovery is going to be nerfed--you spent your time learning how to be insightful and persuasive, but you absolutely suck and gathering your magical energy. Or hey, I actually like those feats that they put out in a UA that gave you expertise in a skill plus an added effect. Feel free to take one next time you hit an ASI/feat level.
Those ideas took maybe 5-10 minutes to come up with. Obviously fine-tuning them will take longer, but it's not like it's hard to come up with cool ways to make a player's concept come to life by faffing around with their race and class but are still balanced and fair to the entire table.
Most of the players I play with have half-orc wizards. They have dragonborn rogues. They have gnome barbarians. The Hat is not only on your head, but in your head. It is a +1. If you really want to be a gnome barbarian, but can't make yourself do it because you only start with a 15 strength instead of 16, then that is a wall you built.
You can play a character with a low primary stat, of course. I do all the time, mostly because I roll badly for chargen. But that doesn't mean it's somehow wrong to be able to put your +2 in a stat that you've trained to improve in.
Absolutely correct. It is what session zero is for. Except now, as has been said many times, there is only one option instead of two.
No, there's six options: Put a +2 in Strength. Put it in Dex. Put it in Con. Put it in Intelligence. Put it in Wisdom. Or put it in Charisma. Actually, there may be a seventh option: break it into three +1s instead of +2/+1.
If this were true. If other players' numbers didn't effect you or the people you played with, then there would be no need to - MUST HAVE - a 16 starting in your primary stat. I mean, if it doesn't bother you or any of your players, then the goliath with 16 strength and the halfling with 15 strength doesn't matter. Right?
I think you need to work on your reading comprehension a bit. What I put in my stats doesn't affect
you or your character.