Robin expressed precisely :
You appear to be ignoring or discounting the fact that many people (perhaps
on lower incomes than you or with less interest in advance budgeting) like to
pay the same every month rather than (usually) significantly less in summer
and more in winter.

I am not one of those, but I find it simpler to know what can expect to
have to fork out each month. As we are still at the beginning of the
warmer, low consumption period, I am in slight energy debt - around
£150 at the last check I made. I don't think any company will be
gaining by the fixed direct debit method of paying, it balances out
over a year.