The Brexit negotiations are supposed to commence in June 2017 and be complete by October 2018 to allow time for ratification before the UK's exit in March 2019.
That leaves just 16 months for negotiation but from this we have to subtract at least 3 months (two Augusts and one for Christmas, Easter etc). That leaves only 13 months which with at best 20 working days a month is 260 days of negotiating time to negotiate everything:
Immigration,
Citizens rights,
Financial questions,
Ireland,
Gibraltar,
Agriculture,
the Economy and Trade,
Customs,
Transitional Arrangements, Passporting,
Justice and Security,
Education,
Science,
Culture,
Legacy issues,
Patents,
Pharmaceutical Regulation, Reciprocal Health care,
Participation in EU institutions, Environmental Matters
And on and on and on. Granted that much work can be done in parallel but as nearly everything affects nearly everything else, that isn't as easy as it sounds.
It simply cannot be done in the time available and attempting to do so will just be silly and counterproductive.