I mean, if you can pay 150k to employ a salesperson to bring in an extra 400k of revenue, why wouldn't you?

Of course, the math kind of goes the other way once you're projecting that your salesperson is only going to bring in 100k due to economic circumstances...

Yes, but there are a lot of ad agencies that try to do Google ads on behalf of clients for huge fees. I'm sure part of Google sales people's jobs is to try and either drive such agencies out of business or get the agencies to spend more on Google.