Reply #2: 50 different procedures...

[Davis_X_Machina]

...in 50 different states, some with two (official and unofficial) categories of write-ins.

50 differing standards on how close the spelling has to be, etc.

And in presidential elections, you're actually voting for a slate of electors who then vote for president. There's no way to do that write-in, unless you write in the name(s) of the elector(s).