Well, that's why you have to tamper with the thermostat. A pre-heater is supposed to bring the work piece to a few degrees below the melting point of the solder - probably at least 20C or 40F to be on the safe side - and the rework gun or soldering iron does the rest. And you want something that can keep it reliably at that temperature.

It makes rework faster, but you can do without it.

NB: I have only seen this hack described; I haven't tried it myself yet. Though I have a couple of pancake/crepe irons waiting to be modded.