It's not so trivial to 'get a mortgage'. Tell me who is going to give someone a mortgage on a property that might be unsellable and which is in danger of falling down?
I really don't know about how it all works in the US, but I don't know if you have been following the 'post-Grenfell' cladding scandal over here?
Lots and lots of apartment blocks were covered in cladding (in order to improve the appearance) which now turns out to be a potentially-lethal fire-hazard. The inquiry about Grenfell _still_ hasn't reported back, but it's widely suspected that the cladding on the Grenfell building was a major contributor to the lethal fire (because it was highly flammable). Large numbers of other high-rise building have the same cladding, and the cost of removing or replacing it is in many cases more than the value of the apartments in the building.
Generally the decision to put the cladding on was taken by the freeholder (often the local authority, sometimes a private company - I suppose that's roughly equivalent to the HOA, though it's usually not owned by the 'owners' of the properties, who only 'own' a lease, not the 'freehold') and because the government hadn't thought through the potential risks and made regulations accordingly, nobody seems to be able to agree whose fault it was that the buildings are covered in extremely flammable material, making them a potential deathtrap, or whether the cladding met the regulations in place at the time.
People in those buildings (the 'owners', i.e leaseholders, not tenants) are stuck with flats that are unsaleable, and in the meantime have to pay massive bills for extra fire precautions, like full-time fire-wardens. While living in a dangerous building. The bills they are facing to fix the cladding problem are in many cases higher than the value of the flats.
It seems possible from what I can see that the problem with buildings like that Surfside condo may be similar. People may have 'signed onto that' when they bought the property, but they probably had no idea what problems there were with the block itself. Getting a survey on a property before you buy it seems much easier if it's a stand-alone single-household house, than an appartment/flat in a large block. Especially if the authorities haven't been doing their job and ensuring the blocks themselves are safe.
I've been watching the 'cladding' scandal with a mixture of horror and relief, as I very narrowly avoided that trap myself (through sheer luck, as it happens). It feels like it's one of the few potential unforeseen disasters of life that hasn't happened to me!
I don't know if the system works the same in the US, but here there are three classes of home occupier - tenants, who own nothing and pay rent, leaseholders, who 'own' the property, because they paid a huge lump sum to do so, and 'freeholders' who have a kind of higher-order ownership and are responsible for maintaining the entire building. They then charge the leaseholders for the cost of maintanance and repairs.
In some cases the leaseholder and freeholder are the same, but for most properties (certainly for apartments) you can't actually buy the 'freehold', when estate agents buy-and-sell houses or flats they are only trading the 'leasehold'.
Flat/apartment owners with leaseholds where the building itself turns out to be unsafe, are in a disastrous situation.
Also, again, I think it might be different in the US, but if you have a mortgage here and the value of the property falls to less than that of the mortgage, you can't just walk away and leave the bank with the loss (which I gather is what triggered the financial crisis when that happened on a large scale), you remain in debt to the bank for the full amount.