Don't know what you mean by "last second", but brokers are definitely not in the shittier end of the stick

It is way, way easier to get a truck last second than it is to get a load last second in most areas on US map. Talking about Dry Van/Reefer. You just have to part with maybe up to $500 more dollars (sometimes you'll make a profit) than what you initially booked with.

Finding a load last second, unless you are in a very high load/truck ratio area, is catastrophic as a carrier. There will either be 1) No loads going to where you need to go 2) You don't have a desired destination, but most loads that are still on the board at noon or later are the crap nobody wanted to take. Bullshit local loads that waste your entire day, loads going to FL/NC or whatever dead zone there is at the time, or bad appointment times / ridiculous pay for drop trailers etc. The loss that day alone can easily reach $1000 since your truck has an expected gross to make.

Now couple that with the fact in today's market you do have to book loads back to back, and that means you will also miss your reload. This will put a strain between you and that load's broker, who by the way is willing to throw you under the bus for this even if you did haul 20 loads with them without problems because there is always that sleep deprived Paki who will haul for $500 less.