Let’s take an actual example.
The PCs are in a gaming establishment. They end up gambling against one of the patrons. Statswise, he is a human noble with + 10 proficiency in gaming tools. Even with expertise, you can’t build a rogue with +10 in gaming until relatively high level.
Of course, the noble isn’t a rogue. He doesn’t have Cunning Action, or sneak attack and he can’t do what a rogue can. While the rogue was learning those cool tricks, the noble was gambling, and therefore is better at gambling than a rogue with expertise.
A player could ask to play a gambler in a future campaign, though the character is pretty useless in the combat, exploration and social pillars (apart from gambling).
From a realism perspective, it is less realistic to say that this NPC who is really good at gambling is also a high level rogue, when there is no reason to establish that he is good at combat, than to say that PCs are adventurers first, and the cost of that breadth is that they may be less good at certain tasks than NPCs that specialize.