The actual answer is:

1) When you are trying to do critical welds you want to eliminate cold start problems. This is when you start welding if the method is such that you can't separate heat input and filler addition, then you get bad welds. With Tig you can heat the weld pool to the point where you add filler independent of the rate at which you add filler. You can adjust heat and filler rates independent of each other and on the fly.

There are a lot of starts and stops in bike welding as you work around the tube, or make tacks to hold parts and reduce distortion. Later you have to burn through the tacks.

2) for the same reasons as above you can control heat and filler as you progress around the tube. You may run into structures like plate drops where you need to pump in a little more heat, or as you progress around the tube, the overall heat in the part goes up, and you need to lower the heat. Constant independent control is a must.

3) There are more filler rods available to work with odd alloys in tubes for TIG.

4) Torch is more configurable, and can be made smaller as it does only one thing, pour on the arc.

5) Shielding is more accurate and adjustable, with more control features, and more structural formats. Custom gas lenses as an example.

All that said some of these limits can be overcome through MIG configuration, though that is not a home shop thing as much as TIG. You are basically forced to come up with custom TIG configs just to get going, while in MIG it is more something on bikes that might be done as part of a robotics set-up.