And because of that the trade off really goes the other way. If you're that tight for space you could put some logic on the part of the logic board beneath the slot and make the logic board smaller, leaving more room for the battery and allowing you to have a thinner battery with the same capacity.
Moreover, "light with good battery life and good performance" doesn't even specify thickness. And quite sensibly so, because that's always been more of a marketing number about who can shave off an extra millimeter than anything that really matters. It's not as if one of them is 7mm thick and the other one is 70mm. And the pointless obsession with trivial differences is how we got this:
https://www.theverge.com/circuitbreaker/2018/5/24/17389220/a...
It might, however, save the company a few cents per unit. At the cost of making the device unupgradable -- which lowers your resale value even if you never do it. So they cost you dozens to hundreds of dollars so they can save a few cents. That's customer-hostile.
And, of course, the other reason they do it is to make the device unupgradable, so you have to buy another one sooner. Which is downright malicious.