Why can’t I keep my dead Logic Board?!

You can look at it from various angles.

First, why do you want to keep a non-working dead electronic part?

Second, if you take your car to your dealer to replace a dead battery or crankshaft, they keep the dead part and give you a new one. That is a standard practice and mentioned in the Apple terms - see an example here:

Legal - Mac Warranty Us - Apple

Your agreement for Apple repair is that they will return your Mac to good working order, typically by exchanging parts for tested, known-good components. The replaced components are NOT returned to you. If the failed components were returned to you, repair costs would have to be MUCH higher.

Your repair comes with two benefits many users fail to notice.

After a thorough up-front evaluation, most repairs are done on a 'no additional risk to you' basis. for example, if the logic board is replaced and they find that the power input board is no good, it will be repaired as well.

Repairs come with a modest warranty of their own, typically 90 days.

-----

Apple reminds you as often as they can that it is YOUR responsibility to make a Backup of your files. They provide the very good Time Machine program, free, and all you have to do is add a drive and tell it to go,

When you submit your computer for service, in the documents you sign, you promise that you have made all the backups you need, so they are fee to replace your drive with a different one.

babowa--

Those are great examples!

Often when you go to purchased an aftermarket automotive part yourself, the cost is bundled with a refundable "core charge". When you return the old part (the core) it will be sent for possible remanufacture, and you save money.

But Apple is not selling you raw parts. Apple is selling you their repair service, with parts included and not available separately.

Thanks; my aim was to have the OP see the Apple conditions which are clearly spelled out in plain terms:

When a Apple Product or part is replaced or a refund provided, any replacement item becomes your property and the replaced or refunded item becomes Apple's property.