Not very practical to haul cans on a regular basis with a flatbed, as you need to be dock height all the time, also, a standard flatbed is almost 4' tall, a high cube container on a regular chassis is 13'5 tall, put it on a flatbed and it's 14' tall.
Yes there are all kinds of specialized chassis. There are chassis that are designed to haul 1 40' can, 1 loaded 20' can, or 2 empty 20' cans. There are 48' chassis that specifically designed to haul 40' containers, the reason they are so long is to comply with bridge laws. In the PNW there are 4 axle 45' chassis that onlu haul 40' cans.
Not true at all, I've run 20' cans loaded to 42,000 lbs, with HAZMAT chemicals, on a flatbed Denver to Houston. Late 90's, early 00's we used to haul an empty 40' can to Idaho Falls on a flatbed, load up 38,000 lbs of satellite parts, and then truck it to Port of Houston. 3 straps over the top and 4 chains for securement.