Hard to make something pro repair and anti theft
Since needing to replace something other than the display is unlikely in the 2-3 years I own an iPhone before trading it in, I'd rather have it worth less for thieves even if that makes it cost more to repair. Others will have different opinions about this.
The problem is that if parts are easily swappable between phones then stolen phones can be broken up for black market parts, but that makes Apple the only source of replacement parts.
There might be a way around this, if the owner of a phone could mark it as "scrap" with Apple somehow (i.e. by logging in with their Apple ID) then Apple could allow the use of those parts. That way thieves couldn't part out a phone, but the owner could choose to do so. That seems a reasonable middle ground.