It's easy... and by easy, I mean it doesn't demand a lot of energy from me. It's a way I can engage with people, even if I'm physically 'shot' enough that just getting out of bed is hard. Just because I may be having a shit fibro day doesn't mean I may not want to be able to interact with people and chat and have fun. I can have fun and RP and 'hang' all while kicking back on the couch in my PJs.
Especially lately... I can't play video games because of the migraines. And I largely avoid going out because of the vertigo. Sometimes I can't RP. But I'm more likely to be able to RP than anything else (even if I've had to bow out a number of times or keep to slow/one-on-one scenes).
I enjoy roleplay/TT stuff, but my shyness can kick in hardcore in person. I have a really, really hard time getting "in character" in LARP, TT, etc. I want to, but I clam up and shut down because I just can't perform, as it were. I get scared. MU*ing is a way of engaging in the activity without the fear, for me.
Lastly, what's kept me around (besides the people <3) is that it allows me to practice my writing. I can try out styles, 'voices,' dialogue methods, different characters, etc.. TS lets me practice sex scenes. Combat scenes let me work out writing out fight scenes that 'flow' visually. Playing male characters helps me practice writing a realistic character that's not my own RL gender. And if I'm ever uncertain about things, I can generally get real time feedback (I've found people are, in general, pretty great if I go 'Hey, I've never played <X> before, how am I doing?').