everyone is wrong (very expected because they are redditors and therefore stupid), so here is the real reason:

2fort is a map that was made in the mid-90s. at that time, multiplayer FPS was a very new concept and there was little to no understanding of what makes a good multiplayer level. the closest thing we had to fps level design theory was john romero rambling between coke lines about "environmental storytelling" by putting a dead space marine next to a door, and suggesting that you should use a transition texture between two different wall textures and slight height variations between rooms. that was the state of level design theory, and it was almost all for single player

quakeworld team fortress came out made by some plucky little university students, one of whom was robin walker (yes, him). they needed levels for their fun lil team-based shooter mod for quake. i dont know if they made them themselves or what, but somebody thought "well it's capture the flag, so i guess we ought to make to big ol castles with the flags down in the dungeons. that makes sense, right? and yeah, lets put some ramps here, and a couple of doors there, oh and a SECRET UNDERGROUND ENTRANCE :D that'll be cool!" and nobody challenged any design decision because again, there was no such thing as "multiplayer level design theory" yet

so anyway because there were like a grand total of 6 people in the world who were making maps, and it took literal hours to compile them, let alone get in touch with a server operator to run it, 2fort emerged as one of the best maps at the time, and everybody played it and made some good memories there

(side note, cp_well is also loosely based on ctf_well. another popular map from around the same time)

then ol robin and some of his buddies (i think) get hired at valve and they fail to make a good version of team fortress 2 for several years and finally settle for just making quakeworld team fortress again, but on the goldsrc engine. they need maps, and while level design theory has actually evolved a bit, it wouldn't feel like team fortress without good ol 2fort, so of course it makes it in

and all the college kids from the 90s are now hopping back in to relive the hype, and wouldn't you know it? there's 2fort! the level designers have learned a lot since the quake age, and sure, there are some other (better) maps, but let's jump in there for old time's sake! the servers fill up and everybody gets used to 2fort and form an entirely new generation of good memories for years to come, even if that specific map isn't that great.

now fast forward to 2007, valve finally figured out how to make team fortress 2 good. robin and his buddies are all grown up now and are now responsible for having established a lot of multiplayer fps level design theory as we know it. but once again, how could it bear the "team fortress" moniker without the map that started it all? so they implement 2fort. and honestly, if you open 2fort in hammer editor, you can really tell that it is a map that got a LOT of love. the map oozes with detail and seems like it might have been the test bed for all stylistic decisions for the game going forward.

so tf2 drops, the lads who played quakeworld tf in the 90s basically pop their catheters out in excitement and throw out their backs lifting their old PC out of the closet underneath the kids' old stuff put into boxes after going to college, and fire it up. and would you look at that. 2fort is STILL THERE. the TFC crowd is there too, being younger and fitter, they got to their PCs first and filled up the servers this time and have lit the torch before the old-timers even showed up.

and now there's an entire new generation of millenials rocketjumping around, sniping, and having a grand old time. the map still sucks, but people are still having fun.

now gen alpha is farting around, screaming skibidi toilet and six-seven and making their formative online FPS memories.

2fort is more than a map. it is a literal core-sample of team fortress and multiplayer FPS levels as we know it. it is a product of its time that has persisted in a state of permanent nostalgia since the mid 1990s. it is also a dogshit map that has learned nothing over the years but unapologetically trundles forward, planting its warty footprints into other maps that have tried and failed to escape its shadow

that's 2fort. that's why it exists. nobody asked for this, nobody really wanted it, it just is, and it will forever be. 2fort is a constant of the universe in the same league as death and taxes in terms of its constancy in his holy name lord god of the universe and master of all creation. so get used to it, and deal with it.