Until you have people who get a yearly bonus. Or 13 or 14 monthly salaries a year, which is quite common in Germany (basically a bonus, but the employee is entitled to it).

Yes but a lot of work is seasonal and/or sporadic. Annual pay smoothes it out.

Most bills are monthly, most paychecks schedules are bi-weekly. To me this is the same issue as hot dogs and buns being sold in different quantities. Whyyyyyyyyyyyyyyy?!?!?!?!?!

I'd imagine that for some jobs (seasonal etc) there is too much variation in a month-to-month basis

At every job I've ever had, I get paid every two weeks. So the amount I make per month varies.

But most pays are fortnightly

I get paid 4 weekly so get 13 pays a year.