I think some people's image of a trade deal is simply that two countries agree to sell "stuff" to one another. That's it. Anything, any time, no tariffs, no ifs or buts, no customs paperwork, just turn up with a shipload, unload it, sell it.
It is not that simple. Each country wants to sell to the other, BUT -
Firstly each wants to sell the things it wants the other to buy, and to have the other country sell the things they want to buy. No point having the other country bring in a whole lot of stuff that you make yourself and then refuse to buy your stuff because they make that themselves.
Secondly, each country has its own system of quality control, and its producers have a vested interest in keeping to those same standards. US chickens being washed in chlorine is one example. They believe that removes possible contamination, but our food standards prefer to tackle that at source with animal welfare regulations on hygeine. Our farmers v their farmers.
Thirdly, no-one wants to see prices undercut (for imports, putting homegrown goods at a disadvantage) or hiked up by tariffs (for exports, making goods less attractive to buyers in the importing country) Each side wants their own goods to be at a price advantage.
So each side argues for quotas, for adherence to their regulations (which are different in every country) and for stiff tariffs adding cost to imports of the things they would rather were bought from home producers.
Add to that the wish to be seen as top dog, enforcing the will of the country with the strongest economy and the least need to get an agreement, (that won't be a UK who have just flounced out of a trading consortium, then!) and sheer bullyboy instincts to show those Brits who is boss. The end result is long negotiations with one side giving a little bit, the other side pushing harder on some other point, agreement almost reached then someone pissing off the country's leader about something quite irrelevant so that months of work are undone and it is back to the beginning.
Not something to be achieved in a couple of weeks!