I play it and I love it, but I think it may have something to do with the developers not fully finishing the game.

It isn’t a bad game necessarily, I think gameplay wise its very good - animations are smooth, combat and stealth is nice and I personally liked the open world but the story is really messy and unfinished and chapter 2 was very disappointing.

Lk

You need to play the previous games to understand why. The Phantom Pain was a HUGE departure from Metal Gear series conventions. Some people loved it and others hated it.

The linear structure and setpieces of past games are substituted for an open-world level design and mission-based objectives that some longtime fans feel drags the game down and is either repetitive or offers players little to do.

What was once a very cinematic and expository experience has been cut down to the player finding cassette tapes if they want to learn more about the game's lore. The previous game Peace Walker introduced cassette recordings, but that game had a fair amount of cutscenes to keep the story moving.

Another contention that fans have is the replacement of longtime voice actor for Snake, David Hayter with Hollywood actor Kiefer Sutherland, who gives off a more subdued performance rather than the iconic gruff and gravel of David Hayter.

"it’s my first mainline MG game"

That's the reason you don't see it. I'm not saying I dislike tpp, but mgs1-3 hit different (a lot of people think so too). In my opinion the game isn't bad, it just could have been much, much better. Some of the (but not all) biggest issues with me in TPP are (no spoilers):

1. Poor story. Again I'm not going to spoil anything, but a lot of stuff doesn't make sense to me. They introduced a bunch of unnecessary things that were never mentioned in previous games and add very little to the lore; they close 1 plot hole while opening 2 new ones.

2. Poor storytelling. Instead of adding more cutscenes they make 100+ cassetess, each being from 1 to 10 mintues long. This just sucks

3. No codec calls. They were always an essential part of metal gear: you could call different people, talk to them, joke around, learn more stuff about the lore and just generally grow fond of characters, seeing how each one of them reacted differently to your actions and surroundings. But in TPP... they are just gone, replaced by stupid cassetes similiarly to cutscenes. Sure, they still talk to you but it's mostly gameplay related stuff like "That's an enemy gunship" for the 100th time. Not tht big of an issue but a disappointment for sure

4. Little to no character development. Fan-favourite characters like Kaz and Ocelot were just there for the sake of being there, doing important stuff from time to time, MC barely talks. The game treats them way too seriously and they don't really feel alive to me, again due to bad storytelling and lack of codec calls/cutscenes. Hey, speaking of characters....

5. Quiet. That woman sniper with ridiculously revealing clothing is an issue by itself.

6. Time gating. It wasn't that bad but I don't see why would they make you wait 1h+ to develop a thing that's essential to completing the mission, same with how end-game levels get locked behind side quests

7. Stupidly hard "loud" missions. Not sure if you've made it to them but that's just bullshit

8. The game is literally unfinished. I can't call the "ending" an actual logical end to the game. The final mission was cut, some plot points were either forgotten or not fully implemented.

Overall, most stuff I've wrote is related to the story which probably won't bother you that much if you aren't a fan of metal gear already. The gameplay is good, missions are fun, graphics are great. Core mechanics are great but like I said the game could have been much better

Like others have said, TPP is just super radically different from the other games in a lot of ways (plus its story is unfinished.) For me, TPP feels a little “too open,” if that makes sense. It’s a wide open world, but with very little to do at least IMO.

Plus Snake’s VA being changed was a real punch in the gut for long-term fans. I can understand Venom being voiced by a different guy since he’s not really Big Boss, but Hayter was just canned entirely. Honestly, I think Kojima could’ve done something cool with the VA’s for good dramatic effect if he played his cards right; have Hayter voice BB in GZ and at the end of TPP when you learn the truth, have Sutherland voice Venom. TPP would’ve been kinda like SoL because it would’ve started as controversial, but by the end that part of it at least be considered a masterpiece.)

I liked GZ because it felt like a decent middle-ground between the old games and open-world stuff since it was a small environment but one which you could approach from any angle. I would’ve enjoyed TPP more if instead of an open world, it was multiple smaller, semi-open environments like that.

Personally, I don't like The Phantom Pain because Snake Eater is my first MG, and I like linear action adventure movies. The Phantom Pain threw me into a big world where I needed to do a lot of missions, waited for a long time for development, and didn't know what to do next.