DIY Bike stands ??s

I have looked at all the DIY bike work stands, and decided on the one that use the pipe clamp, and the steal pipe hanging from the rafters.

My question is why to they always clamp to the seat post and not the bike frame itself?

Also has any one ever done one coming off of a tool bench with the same idea?

If you have pics please show them, and when I am done I will post my pics.

I think I am going to buy the stuff today after work.