But they are all connected airside.

If you mean why there are separate Schengen and non-Schengen zones - that is because of immigration regulations - but even here, you can connect from any gate in either or both parts to any other part, while remaining airside.

Even from the lowcost pier to the "regular" piers is possible without going landside.

(Perhaps you are talking about arriving from an "untrusted" origin, where it is not allowed to bypass security at AMS. In such cases, you are funneled such that you must also pass a security check at AMS - but even here, I thought that this was a transfer security check, rather than being forced to go landside).