Because it's not 5e?
Let me stop right there and say I have no intention of starting an edition war. I'm not arguing that one edition is "better" than another, or BarWrongFun. The point of making this post is for people to have as much fun as possible.
What I am saying is that 5e is a class-based game. It is designed to be enjoyable when a character is of a single class. If it wasn't, multi-classing and feats wouldn't be optional. It works just fine with neither of those rules in play, and if allowed to be, if creatively approached within the basic rules as written, is quite enjoyable even with intricate, imaginative character concepts. As others have pointed out, character concepts can easily be addressed by selection of backgrounds and within-class archetypes. Only rarely have I seen a character concept which required something more, maybe -
maybe - a feat.
Since 5e hit the shelves, I've experienced many different tables. I've seen tables where a fraction of the characters were multi-classed, or had feats, and I have to say in my opinion they
did rather outshine the straight-class-by-the-book characters. Clearly they unbalanced the table. I've done it myself - I have a couple of mechanically-awesome builds which by leaps and bounds overpowered the rest of the table. If everyone at the table was doing it (or was not), I did not notice that effect. The first time I encountered that, I replaced that character with another, completely non-optional-rules character and everything went swimmingly. That made a huge impression on me.
As someone who remembers the early days of D&D, and who gamed through the transition from story-based character-concept communication to mechanics-based, through the time of umpteen gajillion kits and classes and feats to the "book bloat" of 4e, it feels to me like the designers of 5e deliberately made 5e a mechanically simpler, class-based system which has infinitely more in common with OD&D and AD&D than any other version, and that the optional multi-classing and feats rules were tacked on to specifically appeal to those whose enjoyment of the game is predicated on optimization. It's pretty clear from my experience that, compared to games and editions where intricate mechanical character builds were integral to the game, 5e's method is anemic at best. 5e is bad at being Pathfinder!

Further, there is no reason to turn it into Pathfinder,
because Pathfinder already exists.
Which is to say: If character optimization - by which I mean that you're not satisfied unless there's a game-engine-based, mechanical reason for your character to be what she is - is your thing, I respectfully submit that perhaps 5e isn't the game you
really want to be playing. Even with the optional rules, it's not going to be as satisfying an experience as that to which you're used, whether that's 3.5, 4, or Pathfinder. It seems to me painfully apparent that you'd be more satisfied with one of those games than trying to make 5e into a rather feeble approximation of what you want. After all, they already do what you want exceedingly well.
Again, I'm not trying to BadWrongFun anyone. I'm saying that if you want 5e to be Pathfinder, why not play Pathfinder? There's nothing wrong with that!
It's like those kits from the 1980s where you could bolt body bits onto a Pontiac Fiero to make it kind-of-from-a-distance look like a Ferrari. Underneath, it's still a Fiero - for them as like it, a perfectly serviceable car, and wholly unsatisfying for those who really
wanted a Ferrari.
Make sense?
Cheers,
Bob
www.r-p-davis.com