From the point of view of those large customers, would you really trust people working in company X more than AWS/Azure/GCP? Especially since those customers already use other SaaS providers, that probably use at least on the big cloud providers.
There definitely are companies that employ great engineers, follow best practices, and can be on par with big cloud providers, but generally you shouldn't really expect that. In such cases, I'd rather see they leverage managed services, instead of deploying their own servers.
Both SaaS and multi-tenant hardware have massive surface area.
For multi-tenancy, it isn't about trusting AWS/Azure/GCP, it's about trusting everyone you're sharing hardware with.
Cloud products are difficult to setup (AWS in particular). If you can't setup PostgreSQL properly, why are we assuming you can setup AWS properly? Look at the recent Endgame pen testing tool (1)
There definitely are companies that employ great engineers, follow best practices, and can be on par with big cloud providers, but generally you shouldn't really expect that. In such cases, I'd rather see they leverage managed services, instead of deploying their own servers.