The parish council websites seem to have a lot of freedom to run their own standards. Lots of WP and shall we say 'nostalgic' web design!
There are services which will do the work - https://cuttlefish.com/local-councils/ - https://www.parishcouncil.net - are two of many. These have their drawbacks, may not provide exportable data so locking the council in. Some will allow direct access, some require that any new content be sent to them and they upload it.
Some councils will own their domain name, some will not. Then there is the email issue and many will use a gmail / hotmail address.
Cost, especially these days, is also a factor.
I don't see it as a bad thing to have so much variety though.
For all their centralist instincts, the Westminster classes fundamentally don't care about how the provinces go about their business, as long as they keep paying into London and act adequately subservient whenever the Southern classes come knocking. So we have to live with constitutional aberrations like Cornwall and Lancaster.