Because the trade deals are with the EU and not the U.K.
If we drop out of the SM and CU the closest relationship the EU and U.K. can expect is is that of a free trade deal, similar to Canada.
If we can secure that we can trade with the EU without tariffs, country of origin checks and non-tariff barriers.
But negotiating one would take much longer than the time left and it is in any case never likely to be signed.
With regard to the row:-
Trade deals are no longer popular, because of their complexity and growing public wariness means that there hasn’t been a major world deal for 25 years.
Bilateral agreements are the new alternative. But we know where that leads us with TTIP and the sort of noises coming out of the US
The reality of trade deals is that they are vulnerable to every whim of national governments and lobby group. Things get quickly bogged down in domestic politics.
The reality of U.K. trade deal is however that it would be heavily reliant on services which we know are the most complex.
So even if the U.K. has an army of trade negotiators which it doesn’t it is impossible to negotiate a deal within the time left.
If there is no interim deal in place for immediately after Brexit we will face a cliff edge of trade havoc and severe economic contraction.