You don't understand. Of course there can be the odd one that works in a pile of cheap stuff, but there is a higher percentage in this pile that doesn't and it is unpredictable in which way it doesn't work correctly and it is unpredictable when it starts not to work properly anymore.
Thermapen is thoroughly tested, each single one within a certain temperature range. This plus the guarantee gives you security.
A messed up thermometer can ruin multiple brews, and then you need to buy again.... Doesn't pay off imo.