Because HMRC have two completely separate systems/departments
You have the tax side of things which calculates the tax liability for the year
Then you have the collection side of things who actually deal with the payments, debt collection, refunds, etc
Never the twain shall meet. That's as true in real life as it is on the internet. Two different systems. The tax return side of things usually takes a day or two to populate through to the money/collection side of things
That's how HMRC (and it's predecessor HM Inspector of Taxes) were set up decades ago, i.e. different buildings etc., and that's carried on into the internet age
Regardless of two different systems they do meet in the middle, in your tax account. It clearly shows amounts due less amounts paid and a few days after you submit your return it shows you how much you need to pay on the following 31 Jan and 31 Jul.
It can't possibly only be me who has this on their gateway.
Please someone else confirm I'm not an outlier