On top of what the others said, you basically answered it with "However, on the other hand, the engine is only a part of game development". Game dev is one of the most technically complex subfields of software dev, but it still pales in comparison to the total package of disciplines (and skill levels in those disciplines) to make a game.
Compare a basic CRUD webapp to a game. Most webapps won't need stellar art, it just needs to look okay and have some logos or default assets. Games often need to run their own assets for uniqueness, have way more freeform art, you name it. That includes pixel art, the often lowest barrier to entry. Animation? Webapps generally don't need animation beyond some default practice tweening, most games require animation to make things feel smooth and actually give the feeling things are happening, on a vastly higher level than simple tweening of color and position of some flat objects. Audio? Where the average person hates the embedded autoplay video/audio, games generally require at least sound effects, preferably a sound track too. Storytelling? Unless you count the average buzzword-filled marketing video as storytelling, webapps don't need that. Marketing, collaboration, etc. are all factors that come to play as well depending on corporate size, so depending on your goals, you will run against those too.
There is much more too. Psychology can play a huge part if you don't just blatantly copy existing things, and it is far more difficult to map a vague "I like this / I don't like that" than a business requirement. If you don't require animation, in all likelihood you're either creating a game in a genre where other demands are higher (visual novels with higher storytelling and individual art asset demands), or you'll be outcompeted if you don't have something to stand out, in which case one could argue the load is shifted to other disciplines anyway.
Maybe most of all, you generally don't need to develop content in most webapps: you are enabling users to create content themselves, whereas in gamedev, you are more often providing them with content. Of course this comes with exceptions (What about Sims? What about Rollercoaster Tycoon? What about mods?), but most games deliver at least some content developed by the makers of the game themselves. Or you spend days agonizing over algorithms to generate content in a way that makes it fun, making tons of art assets players can fiddle with, etc.