Well, when you consider the example of the explorer's pack, think about what is in it:
Backpack (5 lbs.), containing:
bedroll (7)
mess kit (1)
tinderbox (1)
10 torches (10)
10 days of rations (20)
waterskin (5)
and 50 feet of hemp rope (10) strapped to the side.
Now, a backpack can only hold up to 30 lbs. of stuff. This "pack" has 44 lbs. already, almost 50% more than a backpack is suppose to be able to hold. So, I don't put much stock in the 5E design team for this bit of brilliance.
If you assume instead the bedroll is strapped to the top (common enough), the mess kit and tinderbox are both tied or hooked to a side, as well as the waterskin, then only the 10 torches and 10 days of rations are inside, the full 30 lbs of capacity.
However you look at it, this
is a good amount of gear to be carrying around. 10 days of food (even rations) is heavy and bulky, torches are
not small by any means. a 50' coil of rope
is bulky, and a rolled up bedroll is also a hefty thing really. So, I could see a normal person (STR 10) being hampered by a load such as this.
Here are the rough weights of the different packs in the PHB:
- Burglar's = 47.5
- Diplomat's = 36
- Dungeoneer's = 61.5
- Entertainer's = 38
- Explorer's = 59
- Priest's = 25
- Scholar's = 11.25
Even if you (or whoever) feel the weights aren't accurate that's fine, but I think a lot of players bog down their PCs with too much gear without a lot of thought and then end up complaining about the weight. What you might consider essential, others might consider excessive.
As for your edit#2, IMO the weight associated with the big three scores vs. the little three is overblown in most cases. How important CON and DEX are depends a lot on how much combat your game has in many cases. WIS is used a lot in exploration and some decent amount in social even. While I wouldn't want to have a penalty in these scores, I also don't like a penalty in STR, INT, or CHA if possible.
Too much importance is placed on maxing out a prime score or two if possible, even though a good score (say 14-16) is usually sufficient, even if it makes the game a bit more challenging.