Only build if you have the talent to do so. Early on Amazon managed to attract serious tech talent to the company. That enabled them to build. If a company can't do that then it's better to buy no matter what.
I would certainly hope that if the function is part of what differentiates the company competitively that they'd hire the talent. If they're trying to be the best "X" company and nobody who works there is any good at "X", they should fail. The dot-com era is littered with the graves of companies who thought that just having an idea was enough.
Except almost every software startup in the last three decades was started by folks that were not experts in the domain they eventually became successful in.