Someday the company will dissolve (all of the partners are older than me by quite a bit and there are no plans for new partners) and they haven't clawed back the unvested part for many years. If I can outlast them and they dissolve the company someday I'll get an extra 25k (in today's dollars) + earnings. If I roll over to my current 401k I lose any chance at that money.

Plus I do a backdoor roth every year.