Imagine it is 1992. You have an 80286 and a 256-color framebuffer. You need to fill that buffer with a new frame at least every 100ms to have any hope of having something like "an action game" instead of "a vaguely interactive slideshow".

You have spent the past month or three writing and optimizing some very slick code that draws a first-person view of a maze, with textured walls and floors. Everyone's jaws drop when you show it to them. It takes about 75-90ms to draw a frame on that 80286.

You have next to no time left to run game logic in. Simulating a handful of active enemies and processing player inputs pretty much eat up the rest of the time. But you have this raycasting routine that you optimized the hell out of because it's in the middle of one of the hottest parts of the display-rendering code. So you reuse that as your "is the player aimed at an enemy when they clicked" check. It's good enough, especially since the game's largely set in a tight labyrinth with few spaces large enough that a more realistically simulated bullet wouldn't get to your target in the time between one frame and the next anyway.

When you upgrade your view renderer to handle non-grid-aligned walls and arbitrary heights for your next game, you don't bother simulating better bullets. It's still Good Enough - you're targeting a faster CPU now but you're still doing all the rendering yourself because GPUs don't even exist yet, and your rendering is more complicated now and you still only have about 5% of the CPU time left for game logic.

You start licensing your view renderer to other game companies. ("You", in this story, are pretty much John Romero.) Other people start writing similar game engines but they make similar decisions for similar reasons; "realistic bullet physics" would be nice to have but that's a lot less impressive than "this game can actually render objects with a triple-digit number of polygons while maintaining a decent framerate and resolution".

This situation persists for quite some time until "3D accelerator cards" become something you can expect the average computer owner to have, and you can start spending CPU time on "simulating a more complex world".

(It is also worth noting that 2D games written in this timeframe would generally show bullets as visible objects with an exaggeratedly slow travel time, so the player had a chance to dodge them. Rendering 2D stuff is a lot cheaper.)