The plans are designed to fit every pocket. There are both cheap for new investors and a more expensive ones for experienced investors. So you can choose the one you can afford.

I am also wondering why there are plans... And I cannot just deposit fixed amount.

I don't know if that is the problem...?
Just you have to choose a plan that you can afford and let it go.

I think they are not supporting the fixed amount deposits.

I think they try to make it easier for them to maintain high amount of clients, and also lower working costs that they need to pay, that what I was told in customer service.