But legally under the WA the UK become third country as of the end of the year and thus I suspect legally the UK cannot have an extension even if they now wanted one.
Although they say most is agreed it's the last final legal parts that are important.
There is another complication that the EU states do not really want an agreement in place before they can scrutinise the deal properly i.e. they don't really want to approve it after it's already in place and it is also not guaranteed they every state will agree either and it may require amendments.
They've had plenty of time to resolve these outstanding issues but it does seem that Boris thought the EU would cave in if he held on long enough but the EU said in 2016, 2017, 2018, 2019 and 2020 that they will not agree to anything that compromises the single market .
Apart from that you know that if there was an extension they would do exactly the same as they have so far, drag their feet and spout more nonsense so when that extension was finishing another extension would be considered and so on ad nauseum.