I feel like the rules are like that because "artifact becomes an artifact creature" is way cleaner than "artifact becomes a creature in addition to its other types". It's one of the most used type changes and it saves space. And it's always used targeting an artifact - although this rule technically allows you to turn an enchantment into an artifact enchantment creature, they wouldn't print a card that did that, and this rule only does wonky things when several cards are combined.
I'd actually be fine if they did this with the land -> land creature transition.
I'd actually be fine if they did this with the land -> land creature transition.
Commander:
R Daretti, Scrap Savant
BR Olivia Voldaren
BRG Shattergang Brothers
GUR Riku of Two Reflections
WBG Karador, Ghost Chieftain