There are a couple of problems with the class system in D&D.
1. As was mentioned, some classes come with a LOT more built in lore than others. Which leads to 2:
2. People are still stuck in earlier edition mindsets regarding classes that don't actually necessarily apply to 5e.
Paladin is a perfect example. I've seen numerous references to Steve Rogers (aka Captain America) as the go to archetype for paladins. But, while that would certainly be true in earlier editions, it's not true anymore. There are, what, seven different kinds of official paladins now and each of them is very, very different.
And there's a reason for this. Too much lore in a class means that the class is locked into a single archetype. Everyone who plays that class has to play within a very narrow set of parameters defined by that class. 5e, with it's Archetypes, breaks out of that. If I want to play a First Nations inspired paladin with a spear dressed in doeskin leathers, talking about the spirits of the land and how we should respect our elders, I can. That's an Oath of Ancients paladin, right there.
But, people haven't really embraced this yet. You see it all the time, like
@lowkey13 says, Hard Class or Soft Class. Does a warlock's patron interfere on a regular basis or it is largely hands off? Well, there isn't really an answer to that. It's going to depend on the kind of warlock (how hands on is a Great Old Ones patron really?) and the individual group.
So, while I do think lore is indispensable from classes, I think the bigger issue is that people are unwilling to accept different interpretations of lore and insist that their singular view of a class is the only possible interpretation.