Or..
A lot of people realised they preferred not working during the pandemic and that it improved their MH. Which it did for a lot of people. But improved MH doesn't equal having previously had a mental illness.
And during the pandemic, internet use exploded as did all the social media accounts telling people that if they feel like 'this' - they probably have ADHD/ASD/depression/anxiety whatever.
MN was a case in point, the scantest information in an OP would lead people to say 'sounds like ADHD or ASC, get assesed/get them assessed'. And I mean scant information.
So you had millions of people being deluged with often innaccurate information telling them about all these conditions that they probably have. At the same time you had online advertisement pop-ups saying 'do you have X conditions? then you may be entitled to these benefits, click to find out more'.
Along with all the social media accounts, Youtube channels, Tiktoks etc saying 'how to get points on your PIP claim' .
Plus all the private companies springing up saying 'ADHD assessments, £750 only takes an hour'. Plus other companies you can pay to help you with your PIP claim who advertise their success rates.
So there's another possible reason why the majority of sickness and PIP claimants are for MH conditions and that's because they can't be definitively proven. There's no blood test, x-ray etc, it's almost entirely based on what the person reports and when people are deluged with information about what for e.g ADHD or anxiety presents like and what to say to secure a diagnosis, or a sicknote or PIP. They may exaggerate or fake, or genuinely believe it applies to them.
Then it comes down to the practitioner assessing going on what the person says. Even if they have doubts, they're unlikely to accuse the person of lying due to the very real risk of complaints or litigation so..
That's an alternative perspective. And it was happening before covid anyway, it just accelerated because of the reasons I gave above.