I can't challenge that, but why does the guy who hits 40 homeruns generally get paid so much more than the guy who hits 126 singles?
Because front offices haven't always behaved in a rational manner. Signing someone who mashes to the exclusion of all else usually leaves you with an albatross of a contract like the O's with Chris Davis.
But even thinking that, is this actually true? Most folks who hit 40 homers also have a lot of singles and doubles, etc. Albert Pujols hit a ton of home runs, but he got on base a LOT when he wasn't hitting homers. Barry Bonds got on base a crap-ton when the ball didn't leave the park. And someone who only hits 126 singles isn't very good. But let's look at extreme examples and I'll limit it to traditional stats.
Joey Gallo 40 HR (3rd in MLB), 38 Singles, 24 Doubles, 103 H. .206 BA, .312 OBP, .810 OPS, 500 AB
Jose Altuve 13 HR, 124 singles, 29 doubles, 168 H, .315 BA, .384 OBP, .834 OPS, 534 AB
Who do you think is more valuable? Who do you think is going to get paid more?
These guys have very similar OPS, but i'd argue that their value is wildly different.
The single is worth a lot more than a quarter home run because the act of simply getting on base is incredibly valuable.