> But then, What stops SaaS providers from Hosting in their own environment?

You mean why SaaS providers are reluctant to provide their stack on-premise?

If your main tech is providing a web daemon that "just works" it is simpler (cheaper) to provide (install, update, configure) it on your (the SaaS-company's) infrastructure, as otherwise you have to support integrating your SaaS into the security domain of your customer which is very non-trivial (that's why you often see Team-Support and certain variations of authorization and authentication in the higher-priced plans).

I understand that many customers would like that but I also understand that it is harder (more expensive) to support for SaaS companies. I wonder how long GitLab will support their dual-approach using .com and the self-hosted, on-premise variant.

Yes, a lot of people think SaaS = Deployed on Cloud.

But a significant part of sustainable deployment is the Service part, in Software as a Service

I understand, it is much harder for many SaaS companies to provide that 1:1 attention. Specially when the company would like to focus on the Product.

But there are ways to offsetting it, such as System Integrator, Localized Partners etc.

Large enterprise companies have been traditionally doing the same thing, Seibel for example grew like that. SAP, Oracle, is thriving on it.

But my question was also from the companies purchasing the service? What stops them from adopting SaaS in their environment?