They generally only have four shaped surfaces, four surfaces which have to ground and polished to perfection. It is a relatively simple design problem. To begin to match the quality of that objective, to balance power, field of view, and eye relief, in a design that is well corrected for distortion and color, the simplest eyepiece regularly in use in a spotting scope, and the most common, is as complex as the best objectives. It has six shaped surfaces, in two different elements, and employs two different kinds of glass. Even if you set out to design an eyepiece worthy of your objective, the cost of designing and producing such a lens is often beyond what a manufacturer thinks the birder is likely to want to pay.
Makers know that big fancy objectives sell scopes.
It is very difficult to get most birders excited about eyepieces, or to convince them that they might have to spend as much for the little eyepiece as they paid for the whole rest of the scope. Therefore, until recently, it has been the eyepieces that have limited the performance of high quality spotting scopes. I have often said, here at BVD and elsewhere, that if you are want a zoom eyepiece on your scope, you should shop for the eyepiece that meets your needs, and then buy the scope it fits on. There are still only a handful of truly usable zoom eyepiece available for spotting scopes. There are a few that are good enough so that it is worth buying the scope they fit on.