Because installing a different OS deletes all your programs & data. Sure, you can maybe back it up & restore it, but you can't restore your programs & usually can't use their data with the replacements. Installing a new application doesn't delete the old application.

Of course if the user is technical enough they can figure out how to resize their existing partitions & set up a dual boot. But that's not the default, and that still risks corrupting the existing partition and therefore deleting all their programs & data.