You use inexperienced and/or cheap programmers to build the foundation of your company. Then you bring in experienced folks to keep the barely-functional ball of mud shambling along for the next 10 years.
Startups really need senior people only to hit the ground running; only established companies with seniors can afford to hire juniors.
The thing is after a certain level of seniority, there is really not that much difference so you may just as well focus with someone with 10 years experience who charges a bit less, compared to a 15y dragon.
Plus, senior folks are expensive