Some ideas:
- they had enough money that they never needed to think seriously about maintenence cost, and the sales process was strokg enough to keep customers arriving anyway (look to Oracle for another example of hopelessly complicated installation process but people keep using it anyway)
- at some point someone realized this was actually a feature: the more complicated it got, the harder it became to self host. And from that perspective it is a win-win for the company: they can claim it is open source without being afraid that most people will choose to self host.
I'd say that the architecture that is desirable from the point of view of a large scale SaaS is very different than the one that's desirable from the point of view of a tool that just needs to work for a single organization. And since the SaaS is bringing in all the money, that's where the architecture follows.
> actually a feature
I would guess that for a few people people (e.g. the ones who made the scary visual of rising costs) this is explicitly so, but for most people it's more implied. i.e. I don't think anyone advanced their career with Sentry by making self-hosting easier.
Well I actually did as I was literally hired for that and worked on it for almost 3 years :)