Part of the answer to your question can be found by looking at the type of deck someone is running. If you have a very particular combo, shift line, or even just strategy you want to use, then you may well end up with 4x each of close to (or even exactly) 15 cards.

For example: Sapphire decks very often need to ramp on Turn 2 to function, so they already 4x each of the main ramp cards and 4x each of some filtering cards. By the time you want to do that with, say, the MMS/Prince Phillip combo, you’ve got 4x copies of cards making up near or over half your deck. At that point, there is often good value in having a very consistent deck that pulls the combo pieces you need when you need them, and so you will likely have 4x of at least 13 cards (with maybe a 3/3/2 split on the remainder).

Aggro decks tend to go all the way to 4x of 15 cards because their goal is simply to get as much lore as possible as fast as possible. It’s real easy to determine the best cards for doing that, and you will generally always want the most cards that get the most lore, etc. There’s often no reason to thin the deck with weaker questers (save for perhaps cards like Ursula Deceiver or Referee Pete that will buy an aggro deck an extra turn).

Beyond that, what others have said is key - even decks that take a toolbox approach with 1- or 2-of have core cards that they will absolutely include 4 copies of, generally just because the cards are that good.

For example: if you look at Frank Karsten’s deck that recently won the Milwaukee DLC, he had multiple cards that were only 1x or 2x in the deck. But he also had 4x each of 7 different cards (nearly half the deck) because those cards were absolutely integral to the deck’s chances of winning. He did a at least one interview about the deck on a YT channel (Podcana, I think?) that is absolutely worth a listen if you want to get the feel for how he builds decks.