The dealers do not have crystal balls.

When I bought my used F150, I was towing a 18 foot boat, and not intending to tow a TT (I didn't have a crystal ball either). Turns out, my truck doesn't have the tow package so it has a 5000 lb tow capacity. After posting a question on this website and having someone post a link to a trailer that would fit our needs, (under 5000 lbs with an oven, a Chalet Takena 1860 or 1860BH), we started looking to see if something else more readily available would fit the bill. Knowing the 5000 lb limit of our truck, the first thing we would tell the salesman was that our truck would tow 5000 lbs. The responses we got varied from "You need to buy a new truck before you can buy a trailer", to "Ahh, you're good for 7500 easy", "Those ratings have some fudge factor built in". Of 7 different salespeople at different dealers, 2 did actually listen and, say "well, lets look at what we've got that you can tow". So while I agree on the customer doing due diligence to know what they can tow, the average salesman, at least in my experience, doesn't really care......