Well the answer is because the cost of that software is lower than somebody building the other software.
What happens is that all these SaaS drop in value because it is now realistic to build them internally
Basically I expect way smaller companies popping up competing with the big ones and their offering will be priced way lower because their payroll is way smaller.
While there is no competitor, internal tools will pop up now.
Another great example is open source. I think PostgreSQL being free and usable by everyone is a more economic outcome than every Fortune 500 company building their own database engine. Payroll, ERP, and CRM fall into the same category of being commodity software in a lot of cases.