Having selected a 401k provider for a small (<15 person) company and also for a larger (>100 person) one, I can say that the big names make it prohibitively expensive for small companies to use them. And that expense ultimately comes out of peoples’ retirement funds in the form of fees. They frankly don’t want the business - too much compliance overhead for a small asset pool.

Believe me, I would prefer to have my own 401k at Fidelity too.

I have no dog in this fight, I just know from experience that setting up a 401k for your company is vastly different from setting up a brokerage account, and the reason a lot of small companies end up with off-the-run vendors is because those are the ones that will take the business.