Because they figure there's a bigger opportunity cost in the possibility of the big clients using cheaper tiers than in the possible lost customers who won't go for tiers at the current prices if an alternative is available.
It's really healthy for a company to have a whole spectrum of clients, from people that chip in 2-4$ per month for a minor bump in capabilities and without any support included to big companies that push for exploring new features.
In my opinion, Gitlab is a primary example of feature-creep and it will be the demise of the company.
Gitlab was VC funded which means it had little choice but to take on a high risk/high reward approach. It worked fairly well since it IPOed at $12b while it's last round of funding was at under $3b. Good return for VCs. Of course the stock is down over 50% since IPO but that's someone else's problem.