Again one of the original goals was that services shouldn't require a persistent account. Unfortunately that makes sense in 80% of cases but misses some important ones (Universal Credit, Child tax benefit, Tax).

For whatever reason the people looking at accounts also got fixated on identity (you are Sam Smith) rather than authentication (you have X credentials and have accessed the service before) combined with the UKs aversion to an identity system and central databases (we actually have several) it resulted in Gov.UK Verify, which, last I checked works about 30% of the time.

Ultimately the big departments (HMRC) just built their own thing and created user portals which GDS didn't like... So everyone argued and no one fixed the actual problem.