There's a bigger competition for them; literally thousands of people try out in voice acting school in Japan, some want to move up from there, some because they're otaku (like Tanaka Rie), some just want to get close to their favorite voice actor/actress.
Of course, out of thousand, only few every make it, for every Nana Mizuki or Yui Horie out there, there's like a million nobodies that ended up going back to school, find some other jobs and what not.
As far as "being celebrity" concern, well, seiyuu do have some fan base, but it's not like they have a huge following like Koda Kumi. And they get shit money for just voice acting; hence most of them release CDs, do events, voice in commercial, train announcement, etc. Some even bust tables at cafe like Shiraishi Minoru.
Of course, there's seiyuu that sucks ass as well (like that voice from moegaku go, seriously, the only reason to watch it is if you're really big Aya Hirano fan or something), some improve with time, most don't.