I asked this too, and got the response that I have no clue how long a hospital takes to build, equip and staff. My argument is that 410 billion quid (the current estimated UK cost of restrictions so far) gets a lot done, in timespans faster than anything we ever could imagine under normal circumstances. 2 year specialist training for Covid-ICU, with a 5 year super pandemic wage package gets a lot of people interested in that education etc.
It's not the fact that we haven't done it yet, it's the fact that we haven't even started. The plans have all been short term, and rightly or wrong for the first months of the pandemic that was at least understandable. Since those first months it's been as though most strategies have been based off the vaccines being the one and only answer that would get us out of this and that they would be, so plans only need to remain short term till they "saved" us.