ANYONE can back up up until the day of sale. So you can potentially wait months for your chain to complete and have everything ready to go, have spent a couple of thousand pounds in survey and solicitor fees, and someone in the chain changes their mind. The chain falls apart and everyone loses all the money they’ve put in.
Also Gazzumping is VERY real and you don’t know if the seller is being honest or not .
In other words, an accepted offer is NO guarantee until contracts exchange. In the house I bought, we exchanged and completed on the same day. Took 13 weeks and it was one house. No chain. As simple as a transaction could be, with the exception that we had a mortgage and were not cash buyers. It can be quite a stressful process.
This is England, I understand Scotland has a much better buying/selling process.