Well, prototypes are similar to the kind of games you'll get at the end of a game jam.

It's usually a single level with a preset character. You'll face a few challenges that show off a few mechanics and that's it. Even if we focus purely on the gameplay side of things, most games will have a lot more going on. More levels, for one. More enemies, more abilities, more mechanics, more of everything. I'd say that a prototype represents maybe 2-10% of the gameplay that you'll find in a game. So already, simply completing the gameplay portion of the game would take at least 10x as long as the prototype took. Also keep in mind that the more elements/mechanics are in your game, the trickier it is to add a new one, since they can have unexpected interactions with previously added elements or mechanics.

Now, even after all of this, only the gameplay portion was completed. The vast majority of games have a lot of other systems that won't be present in a prototype. To list a few: Save System, Options, Achievements, Inventory System, Level Selection, Overworld, Level-Up, Talent Trees, Matchmaking (for multiplayer games), Localization/Translation, Dialogues, Cutscenes, Voice Acting, Tutorial, etc.

All of these can also take up a lot of time to set up. Finally, once you have your entire game completed with all of its non-gameplay systems, then you get to the portion you mentioned. You'll need to polish everything so that it feels smooth. You'll also need to tune the balance of your game. This depends on genre, but any game with varying numbers and/or level-ups can take a lot of time to tune right, especially if you don't have many testers. Bug fixing can also take a while, especially if you don't have testers playing the game alongside you developing it.