Is there a reason for the lack of full RTK structures?
Large proteins are challenging for NMR: the more amino acids, the more peaks one has to assign. Peak overlap is also more likely the more amino acids you have, making assignment difficult. Some NMR strategies work well for studying dynamics of large proteins, but as far as structure determination is concerned, there definitely is a critical size past which NMR is simply not the tool for the job.

Flexible proteins are challenging for crystallography: too much flexibility can simply prevent crystallization, you can spend years only trying to get crystals, and once you do manage to grow crystals they might diffract poorly or not at all. But crystallography is often very useful to solve high-resolution structures of individual, isolated domains; this is of great help down the road to interpret a lower-resolution cryoEM map of the full-length protein.

Membrane proteins are challenging for biochemistry: purifying them and getting them to remain stable in solution is difficult.

Receptor Tyrosine Kinases (RTKs) are all of that at the same time! Large, flexible membrane proteins.

Now, with tremendous technological progress of cryo-electron microscopy (cryoEM) recently, such challenging structures become more tractable. Conformational flexibility is still an issue, but unlike with crystallography it can be dealt with to some extent (i.e. it won't prevent you from collecting initial data and getting preliminary results that can guide your next attempt).