Because almost all of us receive paychecks once a month, as the month is the metric used by the government for companies to declare and retain work related taxes. And Brazil is ruled by taxes.
The law here allows up to 48 hours of work each week tho, before it being considered an extra work hour and the company needing to pay the worker and the government and extra for it aside from what its work class/contract says it should pay.
40 hours a week is a very common ammount but many places do 44 or more too (my past job was 46 hours a week).
If you want to have a good understanding of what it does represent, the Switch, Series X and PS5 at their normal prices would be around 2.8x, 4.2x and 4.3x the monthly mininum wage (yes the PS5 is a bit more expensive here).
Right now you can double those Series X and PS5 amounts tho. So if someone wants a PS5 right now and receives the minimum wage they would need to save their money for 9 months without buying food or paying any bills/rent or taxes to have enough to get one.
People in most countries get paycheques once a month, hourly minimum wages still make more sense though. Lots of people work different hours, especially with part time jobs.
So people working 40 or 48 hours who are on minimum wage get paid the same amount? That's... stupid! If you're working 8 hours more than that other person then you should be getting more money than them.
But also, what about part time workers? If somebody only does say 20 hours a week, do they still get the monthly minimum wage?