The answer is that’s just the way English is. Exactly 1 is singular, everything else is plural (mostly).
“On accident” is American English, as a British English speaker I’d consider it a grammatical mistake. The same goes with “I forgot it at home” and similar constructs. However they’re correct American English.
“On accident” is American English, as a British English speaker I’d consider it a grammatical mistake. The same goes with “I forgot it at home” and similar constructs. However they’re correct American English.