Note that wavelength is a noncompositional compound. It does not refer to the length of a wave, but rather to the length of a single oscillation period of the wave: the reciprocal of the frequency. It is technical jargon which does not mean wave length.
Generally we write two words as one when they are used so often and in such a specific way, that the meaning is no longer just the combination of the meanings of the two words.
For example, a blackboard is not a black board. It is not just any board which is black but a tablet for writing with chalk, which may in fact be produced in colors other than black, such as green. Moreover, this is not just an orthographic (written) difference. The word blackboard is phonologically different from black board. It places accent on black and removes it from board. There can be an accent on black in black board, but it is not integrated into the word. It is used for emphasis, as in "I said bring me the black board, not the white one!". The accent could also be on board: *"I said bring me the black board, not the black cord.". In blackboard, there is no freedom to play with the relative stress. An example which shows that this has nothing to do with orthography is (The) White House, which is is written as two capitalized words, yet it is phonetically a unit like blackboard.
Waveheight has not been coined as a technical term probably because English-speaking scientists and engineers adopted the term amplitude instead, which is as precise as wavelength. Amplitude is the degree of displacement of an oscillation from the system's rest position.
If waveheight referred to amplitude, it would be different from wave height, which does not have a clear meaning. Wave height could refer to amplitude, or it could be a peak-to-peak height, which is twice the amplitude. Or it could be something else: the height of shoaling as water waves hit shallow water.