Because crApple tie you in to being able to only get component upgrades through them(authorised dealers included) you are at their mercy and I have never heard of anyone that managed to get this done as every time someone tries they always say it’s not possible and make you buy new!
It should be illegal but they continue to get away with it, if you go down the PC route at least you can upgrade individual components as needed, the only downside is using windows but I have managed to get win 10 stripped down to a useable version.