For a true start up I agree with you, they do not have the time. I am guessing this is what you mean by some companies?

For established companies with legacy code bases older than two years, there is always going to be a getting up to speed period. To pass smart people over because they chose the wrong buzz word for the focus of their last job (or the buzz words recently changed) is a great way to always have people that need to get up to speed on your team.