I think it's just the reasons you said
Even if time is money (a very dubious equivalence), that does not mean time is more valuable than money.
Evaluating applicants is hard, and in some countries it can be hard to get rid of them after a short trial period.
So instead, you take on interns, that way you are pretty confident which ones are good, can easily get rid of the rest, and don't have to pay them much while they learn your system (which they have to anyway, even if experienced).