Because the only rational reason to spell it Artoo-Detoo and See-Threepio is if they got paid by the word and they wanted to bolster their paycheck? The alternative that they somehow thought this was a good idea is even worse.
I'm pretty sure it was so they would look more like real names, because Star Wars treated robots (droids) like actual characters which was rare at the time.
Not just 70s and 80s, even 90s and 2000s. I grew up in 2000s thinking it is called Arturito, and I'm sure a lot of people (mostly non fans) think the same.