It is all about the upfront costs and maintenance.
It takes some serious capital and talent to build a high-availablility data center that SaaS run on.
The software is just icing on the cake. You did not include the cake, the plate the cake is on, the table the plate is on, the chair at the table, and the roof overhead. All of those other components are needed and they have costs which must be paid for everything else to work so well and be so easy to use.
Come on, it's clear OP meant SaaS in general, not Amazon that also does SaaS as a value-add to its cloud infrastructure.
No SaaS provider is building their own datacenter. They usually don't even run their own servers...
True, but SaaS still runs on cloud infrastructure so the costs are still there: you are either Amazon, Microsoft, etc. and build the infrastructure yourself or you pay them to rent time/space from them.
The base costs for the infrastructure do not go away which makes it unlikely for SaaS costs to race to the bottom.
You can bring the costs way down. Their margins include a pretty insane amount of features that not everyone needs. There is a lot of meat there if you have skills in the area.
Making/saving money via DIY compute is an area ripe for improvement via better vertical integration.
The big cloud providers simply do not have this market cornered even with this mind share that prevents perfectly good engineers from seeing the path plain before them.