Neltharion was never "all about strength," even before his madness and corruption into Deathwing. His main offensive thrust against the Legion in the War of the Ancients was to create a powerful weapon he could use against them as opposed to enhancing himself or taking the Aspects' combined powers for himself and increasing his own personal strength, after all. As for the dracthyr, there are a number of reasons why they don't have the physiology or appearance of the drakonid - from differences in roles to limitations on how much Neltharion could alter the essence of his baseline while still maximizing potentiality and adaptability, which is the reason he couldn't use the drakonid for his goal of accessing multiple Aspectral powers in a single entity in the first place. Presumably, if Neltharion could turn the tarasek into literal dragons he would, but that doesn't seem to have been an option. Neltharion isn't a Titan, and it's likely that kind of evolutionary power to enhance a living being is beyond him. He likely had limitations on what he could work with, and adding Aspectral potential to the tarasek baseline may have taken options like robust physiques off the table as the goal was to hyper-specialize for magical draconic potential, not combat prowess.
Non-Evoker dracthyr were likely considered fodder for protecting the true Evokers, the elite of his efforts. And all of the above also discounts the notion that despite being lithe and spindly, the dracthyr could be immensely strong due to their magical nature. A common field ant can lift up to 50x its own weight and endure pressures up to 5,000x its weight, after all; and they have no muscle mass to speak of either. Perhaps the dracthyr are simply built in a way we can't even begin to compare with terrestrial examples, being what they are.