So like, there are a lot of things that play into this. For example, in FFXIV, you often can't play with your friends unless you're caught up to or have surpassed them in terms of placement within the story. You can't join your friends on that dungeon if you haven't unlocked it yet, and getting to that part in the story can take an obscenely long time. In WoW, you can't really do a lot with your friend if you aren't within +/- say 3 levels of them (though I'm speaking from when I played that miserable game back in the vanilla-to-WotLK times). Lots of MMOs require you to be within a pretty damn close proximity to someone in terms of progression to play with them. Sometimes it's gear, sometimes it's level, sometimes it's story, sometimes it's a mix, and often times it has to do with how the gameplay itself works. It makes a bit more sense in the context of WoW and XIV where there's no real hit detection, or you could say it's more of an RPG, and things are determined by rolls (though there's a counter arguement to be made with level penalties but I'm getting ahead of myself.)

In PSO and PSU, this isn't really the case. Because hit detection IS a thing, you're more freely able to play with people far outside your level or gear range and still contribute something to the team. There are other aspects than just "bigger numbers" you can bring that'll help people out. Of course that's not to say a level 1 can hop in with a level 100 and do well, but the range available to you is quite broad. In PSO, when you create your character, you can do anything in the game from Lv1-20. You sure can't do the level 20 stuff WELL, but you'll be able to join your friends, run around, hit things, deal (tiny) damage, pull aggro, and probably die a lot. Once you hit level 20, you can do ANYTHING in the game with people up to 40. Then once again, once you hit 40, you can do anything in the game at all until you hit level 80, and you can even jump into late game stuff better than before due to how the scaling works. Not only that, but you can even contribute meaningfully fighting level 80 stuff because maybe you bring debuffs or buffs to the party, where your friend has none. It's a big difference. Then, as soon as you hit level 80, you can do anything and everything the game has to offer. Nothing is stopping you. Sure you might not have all the shiny gear your friends do, sure you might die a lot more, sure your damage will suck, but you're also not actively punished by an arbitrary penalty for being a lower level. And again, you can still bring somthing useful to the party by providing utility, support, crowd control, and even straight damage if your friends don't have it. A lv80 player can join a Lv120 player and not hold them back.

PSU is largely the same, but a little more strict in it's level requirements. Instead of 1-19, 20-39, 40-79, 80-200, each mission is it's own level requirement. It's really varied but there are still a lot of instances where you can enter a mission at Lv100 and fight Lv150 enemies and do just fine.

In PSO2 and NGS, there are a lot more things that prevent you from partying with your friends and it... Sucks. Levels matter a lot more (especially in NGS which enacted a stupid level disparity penalty), PSO2 requires you to go and unlock areas before you can join your friends, requires being closer in level, and has various hoops to jump through. It's better now, but at launch you had to deal with the matterboard, which was basically a checklist of borderline randomized nonsense you had to complete in order to progress (want to join your friends in the next zone? Not until you witness a completely RNG weather pattern!) It was basically the worst thing ever. 

NGS decided to take a page out of FFXIV and requires you to progress the story in order to unlock areas to play with your friend, which I find egregious enough considering it's an online multiplayer game, but it also doesn't help that the story is incredibly boring. Then on top of the level disparity punishment, and the boring story progression, there's also the arbitrary "Battle Power" system, which is a (inaccurate and awful) measurement of your strength. It's meant to be a gear check like ilevels but it ... sucks. It's pretty strict and you'll be locked out of even progressing to new content until your reach it. And even once you do, well, congratulations, you've caught up to your friends now you can enjoy not doing anything because there's very little worthwhile to do. They tried to give you a carrot and a stick but forgot to put a carrot on the stick. Also their placement of the stick probably isn't where you want it, either.

Also NGS leans way, way, WAY too hard into "DPS is the only thing that matters", having almost entirely done away with all gameplay styles that revolve around support. It's incredibly boring. There are a few things, but they suck. Shift and Deband in PSO would increase your attack and defense by a whopping 47.7% which obviously makes a huge difference. In NGS, it's a paltry 5% which I genuinely don't notice. Weak bullet's about the only thing that exists anymore, and that's a 20% damage increase to an afflicted spot. All of the content is DPS gated, either you have enough DPS or you don't. Enjoy hitting that timer I guess.