They have tried in past - and they scrapped them - but people are still under them even now.
https://www.independent.co.uk/news/uk/crime/ipp-prison-living-nightmare-lords-debate-b2501376.html
They include tragic cases highlighted by The Independent such as those of Wayne Bell, who has served more than 16 years for stealing a bike; Thomas White, who is still in prison 11 years after he stole a mobile phone; and Shaun Lloyd, who fears he could be hauled back to jail for the fourth time for stealing a phone almost 20 years ago.
...
IPP sentences were scrapped in 2012, but not retrospectively, leaving almost 3,000 inmates trapped in prison with no release date – with more than 700 of them having served 10 years longer than their minimum tariff.
More than 80 IPP prisoners are known to have taken their own lives – including the seven whose self-inflicted deaths were revealed by The Independent last year.
...
Under the controversial sentences, which were introduced by New Labour in 2005 but scrapped seven years later, people were handed a minimum jail term without a specified maximum term.
After completing their minimum tariff, IPP inmates must apply to the Parole Board and meet stringent criteria in order to be released – including not suffering from mental health problems – to prove that they are no longer a risk to the public.
It seems some young men who did petty crime have now in some cases spent longer in prison than murders and rapists. Plus recent early releases as prisons full didn't look at these people.
I like idea dangerous men off streets for as long as possible but perhaps it harder to judge or perhaps we actually need to get serious about rehabilitation generally.