Tye typical reason for long closings, at least in the US, is sale contingencies and financing. If sale of your existing home is contingent on closing on your new home, and that sale is contingent on another closing, things can take awhile to sort out (and not much time at all to blow up).
Financing can also take quite a long time, especially if you have a non-traditional income stream (i.e. you're a startup founder).
A buyer who doesn't have the money now, doesn't have the money. The only reason to humor such people is if you have the asset priced too high. In that case, those might be the only people interested...
In the UK, ordinary families don't have access to large amounts of ready cash - the money they intend to spend on your house is currently tied up in their existing house. So if you want the deepest pool of buyers (& hence to realise the best price for your property) you have to accept being part of a chain of house buyers, all of whom need to perform a dance where they simultaneously sell their houses to each other in order to raise the cash they need to buy the next property in the chain.
It should be unsurprising that this process is widely regarded to be more stressful than anything other than death in the family or divorce.
Financing can also take quite a long time, especially if you have a non-traditional income stream (i.e. you're a startup founder).