Simply put? The FDA.
The thermometers you're talking about are thermocouples. Thermocouples aren't crazy hard - you can get really precise ones for not a shit-ton of money. But then you have to get them FDA qualified and if it's something that can go in an orifice - any orifice - it's in FDA hell-world. So you take a $1 thermocouple, a 50 cent display board and a 20 cent plastic body and it's suddenly a $30 digital thermometer because $26 of that is the paperwork and testing necessary to get it FDA-approved as a healthcare product.
If you go with a not-stick-it-in-a-hole thermometer you capture more of the raw technology and less of the regulatory oversight. Sterilization and contamination protocols for a contact medical device are substantially more chill than for anything you put in your body.
I've got a Thermoworks Thermapen. It'll read to a tenth of a degree in half a second with a NIST certification but it's not healthcare approved. We use that exergen jobby for the kid; you gotta swipe your forehead like six times and take the highest number.