Why do I have to have a payment method to get free app’s?

This may help explain—>If you can't remove your last payment method or use no payment method with your Apple ID - Apple Support

It's because many free apps have in app purchases and/or add free paid version etc.

Actually it has more to do with if you have any subscriptions such as Apple Music, Apple TV+, Apple Arcade, or any other third party subscription (e.g. Netflix, Hulu, Disney+, etc.).