Because former PM David Cameron and his chancellor George Osbourne committed Treason in the 2010 SDSR and axed 5000 sailor, 5000 airmen and 30000 troops, all in skilled professions. That was a big hit to the armed forces and they are still struggling to recover. Skilled marine engineers are hard to come by, as they can earn a lot more in the private sector, and even if they join the RN they are often temped away after a few years with valuable experience under their belts.
Successive Governments of all colours have engaged in 'salami slicing' of the armed forces since the end of WW2, and there comes a point when further cuts only harm the capability of those forces to defend the Nation. In 82 we sent a task force of over 50 ships south and it was barely enough. Currently the RN is less than half that size, although individual ships are far more capable, they can only be in one place at a time.
The government has belatedly realised the error of their ways, even though it means going against their mantra of Tax Cuts for Billionaires' before all else, and this was reflected in the decision to fully commission PoW as well as the decision to cancel the 5 T26GP frigates in favour of the much cheaper T31e, with the express intention that numbers may well be increased if they can be delivered on time and in budget. Ordering more River class OPVs and retraining the older vessels is an attempt to bridge the gap in hull numbers too. Constabulary duties are really stretching the RN right now, with patrols being carried out by OPVs and RFAs that normally require a frigate or destroyer. Funding has loosened up a bit, but recruiting and retention are still a big hurdle. Replacing the 'lost 5000' isn't something that can be done overnight.
