It's not just a phenomenon linked to the Covid timeline.
Around 10 years ago it was enormously difficult to be awarded DLA for purely mental health reasons. Approximately 80% of all rejected applications were eventually awarded once they had been pursued through mandatory reconsideration and then Tribunal. These were only the rejected application that were actually challenged though, not ALL rejected applications. So the long and short of it is that the majority of people who absolutely should have been awarded DLA were not, and at best, they'd end up on JSA and be subject to the strictures that come with that.
Over the years, increased exposure of just how unfit for purpose the Capability assessments were, and scrutiny of the processes used by ATOS served to temper somewhat the DWP's thirst for finding people fit for work who clearly were not. Of course, this pressure for "targets" came from Government, which is in itself utterly beyond ridicule. The idea that you must find people perfectly fit for work lest doing otherwise takes you over a completely arbitrary threshold. Complete and utter nonsense to begin with.
So the reality is that this picture of a vastly increased number of disability-related claimants is a bit misleading, because we're not "more ill", the Government was just getting away with finding many more ill and unfit for work people fit for work, until the nonsense of how they were going about this was laid bare for the scandal it was and they rowed back from it.
The numbers claiming these benefits now are a fairer reflection of reality, and more akin to where they always should have been, not some sort of indicator that vast numbers of people are "at it". Of course, the Tories don't care about that, and would far rather have the public blaming the unfortunate for their own misfortune, rather that question why we have a society that creates huge numbers of mentally ill people who are incapable of work.