I think that the figures for the UK are that 'most' PPLs will fly 50 hours after getting their license before stopping.
As MLS points out - its probably due to time & money.
As a student you are mentally committed to giving up large chunks of time and money to get your license, but as a new PPL, all of a sudden you are on your own. There is no incentive to fly, no exams to pass, no lessons booked.
As a PPL, it may be that you can't afford to fly as often as you did as a student, so you decide on a sensible budget, lets say £150 per month, which might be 1 - 1/5 hours club hire.
So after flying friends and family around and visiting nearby airfields, then the novelty wears off. After all, an hour will only give you 25 minutes radius, by the time you've done the checks and queued at the hold.
So whats the answer? I wish I knew. As far as Flying Schools are concerned, so long as there is a constant throughput of students, they are not going to be particularly bothered about what happens to PPLs. If the number of students drops and the aircraft become under utilised, then they might start to worry.
The gliding fratenity, I think, have a series of goals that can be worked towards and it has been suggested that a similar scheme be introduced for PPLs.
I reached the 'PPL + 50' last year and was frankly getting bored. But I was in the fortunate position of being able to afford to buy a share in an aircraft and now I'm enjoying the benefits of cheaper flying.
What we need is a mid-point supplier, someone who is not a school, but isn't selling a share. Someone who will rent an aircraft to a PPL cheaper than a club, but without the large fee of a share.
I would recommend to all new PPLs that they try and find another PPL to share their flying - its so much more rewarding.