[larrik]

My theory:

1) They generally don't make enough profit to save up and then cover the investment period of developing new unproven software (which can be years).

2) If it's too similar to work they've done, former customers may be able to claim some ownership and eliminate the profit.

3) If it's too dissimilar to work they've done, then it's just a shitty startup with no funding.

4) Even if all that worked out, it's wildly impractical to work on client projects and your own projects with the same staff at the same time. So you either need to do one or the other.