Why is TIG welding more expensive than MIG welding

Update: 25-09-2020

Well, for starters, good usable TIG equipment is typica […]

Well, for starters, good usable TIG equipment is typically at least twice as expensive as comparable MIG equipment in the same power and sophistication range, and this price differential continues up until you get into the high-end "professional" machines. Advanced waveform shaping, good arc characteristics and ample power reserves don't come cheap, regardless.
Secondly, the raw skill level required to make usable TIG welds is much higher than MIG, which has been promoted as a 'point and squirt' solution, even though the skill and training to make good welds on a variety of materials is much higher than advertised. Making "good looking" welds with MIG is relatively easy, making sound, strong welds is not... MIG gives you the ability to make cosmetically acceptable welds at the highest rate, and the process lends itself to automation (welding robots) which makes it atractive to mass production assembly lines. As has been already mentioned, MIG is typically a minimum 4 times (and can be 10 times or more) faster than TIG when linear speed or weld material deposition is important. Where TIG shines is in weld quality due to it's heat control, high purity of atmosphere in the weld zone (especially when shielding gases are applied to both sides of the weld (back purged), wide choice of filler materials available as well as the ability to make autogenous ( no filler material added) welds, and typically better ductility of the weld zone with minimal change to the metallurgy of the base material.

The things required for a good weld that is sound, strong, and attractive don't really change between processes;
1) compatible materials - they have to be similar enough to actually fuse, as welding is a fusion process
2) good fit up - sloppy fits mean that more filler is necessary and the likelihood of a weak join is greater due to temperature differentials and atmospheric contamination of the weld zone
3) cleanliness - contaminants make weak welds and are points of failure
4) joint preparation - cleaning off surface oxidation and mill scale (for steel), bevelling the joint to insure adequate penetration when appropriate
5) adequate shielding gas - this also includes ensuring that the local environment is calm enough that your shielding gas isn't blown away. Back purge where possible or necessary. Sometimes all that is necessary is a piece of ceramic tape applied to the back of a weld seam to ensure good gas retention and coverage.
6) appropriate heat applied to the joint/weld - realistically 3/16 of an inch (2.5-3 mm) is about the limit for most 130-150 A welders. With good joint prep you might be able to stretch that another 1/16 (1 mm) or so, but you won't (or shouldn't) be considering building bridges or trailers with that $200 MIG welder off the internet. Too much heat (current, usually) is usually readily appearent when you blow holes in the base metal, but too little will give you a "cold lap" weld which may look ok, but have little or no fusion, and hence strength.