I have always been an "alround" gamer. (whatever that is anyway)
So basically I have been trying to play many different games of any styles, the obscure and the mainstream ones. And for many years I have been really enjoying doing this. But unfortunatelly in more reccent years I have lost more and more interest in most main stream games. Because I simply couldn't enjoy them anymore. They were simply too time consuming and ... boring... because of all that filler content in them. 50 hours of always killing the same enemies in a FPS (single player mode!), 100 of hours of doing the same stuff in a japanese RPGs and don't get me started on the quests of MMORPGs. Simply the whole "play for 100s of hours to collect everything" philosophy is something that absolutely turned me off many modern games. I wanna enjoy ALL the time I am playing the game. I want to put it in and instantly get emerged, no unneccessary tutorial levels and endless stretched levels with just the same repeating structure just to let the game last longer.
While one may think that shmups and other "classic" genres (as called above) are just as repititve. They may actually really be repititve in some way, so repititve is not really the right word to use here. Hmmm... I am just talking about a general interesting and exciting gaming experience. Like chess is repititve because you always have the same pieces and moving rules, but yet you have to be careful on each game.
And for me, shmups is one of those genres which still provides enough of a challenge and a burst of pure gameplay to keep me coming back and interested in a long term. Also some other arcade style games or multiplayer games.
But if you wanna "try" to sum up why people play shmups while they could play so much else
cough, garbage... 
. Though I do understand that what is "fun" is different for every person. And so some people find WOW fun to play, which is for me beyond any understanding at all! But that is just my point of view, I realize that those people probably enjoy what they are doing. So, I am just asking you to, please, not write something like people find it cool to be retro, or people like to collect, or people which are immersed into a certain community like shmups. The people playing shmups do that because they simply enjoy the gameplay shmups offers to them. As weird as it may seem to someone who doesn't like these games. It kinda sums up to that you can pick up and play it for a few minutes, but come back to master it. And the process of mastering is a FUN challenge to them. Many on here would still play shmups without the community too.