>, how come sales people are unable to do things for a fixed monthly salary?

You have to separate out 2 different ideas of the "theoretical idealized salesperson that works for fixed salary" -vs- "real-world salesperson that works for variable commissions".

The businesses that have attempted to pay fixed salaries for salespeople end up attracting incompetent salespeople who can't sell. They become a negative cost on the company's payroll because they can't bring in any revenue. In contrast, the high-performance salespeople (the "rainmakers") are attracted to the variable high-commission, because they know they have the hard-to-find skills to actually sell and bring in the money. If a salesperson has the skills to get a customer to sign a contract and pay money, they have the leverage to get a percentage of that.

Developers, db sysadmins, tech support staff, etc are not in situations to directly influence and shake the hand of a new potential customer and convince them to write a check.