So we agree that taxxons would be impractical due to their feaces and you would have to use technology to dig a whole.
As to why that technology is human rather than advanced yeerk technology:
answer the yeerks are parasites and in the same way that humans exhibit 'predatory' behaviour, yeerks exhibit parasitic psychology: they don't really have original ideas, so they use human creativity/creations, indeed it seems that yeerks take on many of their hosts qualities (although they shouldn't as i have said earlier-the yeerks should do quite well if we gave them undesirable humans) such as feminity (edriss) hubris (esplin), the sheer slowness of many hork-bajir controllers (as seen the yeerks tendency to gabble words) and in more obvious ways such as Edriss being an awful driver.
The yeerks are a demonstration of the difference between genius and simple intelligence.
Comparably on Earth:
The Japanese, until the Meiji restoration deliberately isolated themselves as their authority could be challenged by outside influences (which is why the last samurai is so stupid=it effectively sided with the Samurai who were a caste who knew that modern weaponry challenged their elite status), when Captain C Perry arrived on the black ships he caused the opening of Japan, the japanese were apparently ashamed at the humiliation of losing to western/foreign technology and realised that they needed to develop Japan into an industrial and military power. So they set their navy training people to study with the English (who had the best navy) their army with the french etc. and when the french lost the franco-prussian war, the japanese withdrew their army training people from France and sent them to Germany, as the they decided that the German army was obviously much better. So the whole of Japan's development was copying off other more advanced (western) nations, they defeated the Russians (the most backward westerners) the first time any Asian nation had defeated a western power (although this pushing the analogy Hork-Bajir=Russia). They then proceeded to lay waste to Asia. At the beginning of WW2 Japan had the most advanced Airforce of any nation, but cut off from original thinkers and consumed by fascist ideology they had not improved by the end. Today the Japanese continue on miniaturising, aesthetising, improving and perfecting foreign inventions: Americans invent the car but Japanese cars are smaller, prettier, with more gadgets, fuel efficient and all around better. Likewise the yeerks borrow alien technology (the shredder) and improve it (the dracon beam which is more painful)..
Obviously thirty people will jump on this analogy, and I am aware that I simplify things a lot. but it is something to consider.