Same reason that there are more roles for older men and fewer roles for older women.
Same reason that network execs seem to think that the word is populated by teenagers with token adults on the fringes (Zoey 101, 90210, that Aussie show about Mermaids, etc.). Sometimes these teenagers/young adults are completely sex-crazed (the OC, The Hills, BBC's "skins") and seem to not have jobs. Or go to school. Or do anything other than talk to each other. And go to bed with each other.
Same reason that supposedly a *very* modestly talented lounge-singer who, as a benefit of singing full-time at the hotel, gets to live in a two-room suite at that posh hotel along with her twin boys.
Same reason that even though women outnumber men they have fewer roles on TV.
Same reason that one season there can be no primetime serial dramas.
The next season a serial drama is picked up by ne of the networks.
It is a big hit.
The season after *that*, all the networks are awash in serial dramas.
Same reason that one season there aren't any reality shows.
The next season a reality show is picked up by one of the networks.
It is a big hit.
The season after *that*, all the networks are awash in reality shows.
There is even a reality show network on cable.
Same reason that one season there are no Westerns.
The next season....
agnes!