Why can’t the battery be changed alone on MacBook Pro?

Apple decided to glue the battery onto the underside of the Palmrest & Trackpad making the Battery a part of the Top Case/Keyboard Assembly (along with the Touchbar & Speakers for the Touchbar models). While the Top Case/Keyboard Assembly is very expensive.....Apple offers a Battery Only repair price which is about half the price for as it would have been for a Keyboard or Touchbar failure as long as the battery fails the Apple Diagnostic.

For the 2016 to Mid-2019 models, getting a "free" new Keyboard out of a Battery replacement is a very good deal due to how terrible & prone to failure those Butterfly Keyboards are....getting a new one is a great bonus. IIRC the MBPro 16" (2019) model was lucky enough to get the old style scissor mechanisms for their Keyboards. But I do agree it is a terrible design.

In the US the Battery Only price for the repair is $249 according to the following Apple article:

Mac Repair and Service - Apple Support

FYI, I personally would not spend any money on repairs to any of the 2016-2020 Intel Macs with USB-C ports for multiple reasons. The money spent on repairs is better put towards a new laptop. However, a battery replacement is one of those borderline exceptions as long as the laptop is in otherwise very good condition. Unfortunately the MBPro 16" (2019) model is known to have lots of Logic Board failures.

It’s true, you cannot simply get a battery change for the newest MacBooks. Batteries are glued into the assembly and it’s much easier and faster to replace that assembly than to unglue the battery.

It is what it is, my friend.

Assembly replacement is very common in any number of devices and machines. Component replacement or repair has gone the way of the do-do in so many consumer products. Apple’s newest computers continue the trend.

FWIW, I agree with HWTech. I would not spend the money repairing a 6 year old computer.