For shorter holes traditional mechanical methods work just fine. If you are going to build a giant excavator you don't waste time making shovels for gardeners. The problem drilling deep into ground is that the power source on the surface of earth and drill bit deep underground are connected by long floppy noodle while the hole is getting crushed from the sides by bunch of elephants. It is difficult to transfer rotation from the motor/power source at the top to the boring head, and reinforce the walls to prevent them from collapsing, having whole thing heated to few hundred ℃ doesn't make it easier on hardware.

In case of something like underground tunnels these problems are avoided by having hole big enough to fit the drilling machine as well as all the equipment and crew to reinforce the walls with concrete.

The fact that people have made a way to drill few hundred to few km using mechanical means is already an engineering marvel. In the context of everyday manufacturing beyond the hole depth to diameter ratio of 5:1 things already start to get more complicated. With more specialized techniques you might get 10:1 - 100:1. A bit easier for softer materials like wood or if you don't care about precision. But for deep underground drilling we are talking about ratio of thousands to 1.

It's not like they are not making tests at shorter depths. Once technology is sufficiently developed it might also trickle down to some shorter few km holes if geological conditions are right. Although probably never for something like few dozen meter water wells or making a hole in concrete at construction site. Not sure how well it works in soft dirt. Who knows about distant future, we now have relatively cheap desktop laser cutters, laser pointers, measuring equipment, microwave ovens, but those were not the initial products when developing those technologies. On the other hand some tech like wire EDM has remained niche manufacturing technology, even though modern electronics and software could allow making it much cheaper.