Why do women have to start birth control pills on their menstral cycle/?

Supposedly to keep them on a regular cycle each month.

My case was different. I started on a different day because I was having ongoing bleeding at the time and started the pill to stop it.

Honestly, they don't.

The pill COMPLETELY overrides your natural cycle and imposes an artifical one.

The period that you have while on the pill isn't a natural period at all, it's a withdrawal bleed caused by a drop in hormones during the hormone-free week. It's not medically necessary to bleed once a month, it was included when they first designed the pill to make it seem more natural, so more acceptable - a marketing thing, not a medical one.

There are two reasons that starting during your period is convenient.

1 - you know that you aren't pregnant.
2 - your next withdrawal bleed will happen around the time that you would have expected your natural period to happen.

If you know you aren't pregnant, and don't care when your next bleed will be, then you can start whenever you like. Most pills start with a weekend so that you will bleed during the week and keep your weekends period-free.

Because part of birthcontrol is regulating the periods.

If you had a period 2 weeks ago and then started your pills at the beginning of the pack then you're messing up the cycle your body already is on. Because then your body is going to be told to go 6 weeks without a period instead of the typical 4 (in that example.) So its just to keep your body on what small schedule its already on.