They are popular because they were permissive and dead easy to use in… 2008 or so. Indie studios adopted Unity, many went on to great success, and it is now a product with a huge userbase, tons of tutorials, and an industry full of experienced Unity developers.
Unity is successful, entrenched, known, and makes money.
And this is the worst possible situation for a product to be in. The term is “enshittification” and basically means the owners are incentivized to sell out, while new owners are more incentivized to add new ways to extract value than to improve the product meaningfully. This is a death spiral that no products recover from.
It’ll just get worse until in several years all that is left are some patents bought by a patent troll who sues the everliving shit out of anyone still using Unity.
Anyway: Look up Godot. It’s a modern version of what Unity was when it was young and pretty.