I think you are having difficulty finding the answer because you are perhaps not asking the right question(s).
...why are more efforts not made to improve their function?
How would you support the assertion that more isn't being done to "improve their function"? The fact that you don't know about something doesn't mean it's not being investigated.
...knowing that the body already has a tube designed to do the same thing without the risks why are more efforts not made to improve [e.g. by stenting] their function?
You will not find any support for your proposal because stenting is not a benign procedure, and the risks of stenting the Eustachian tube far, far outweigh the benefits, as well as the risks of the alternatives. At least tympanostomy tubes have a physiological comparison in ruptured tympanic membranes.
First, the Eustacian tube (ET) is normally closed in people of all ages.
It was Toynbee, in 1853, who concluded from experiments on himself and from the tendency to swallow while descending in a diving bell that the eustachian tube is normally closed and opens only during swallowing.
The ET also opens with yawning. Compare how much time is spent in the resting phase of ET function (all the seconds or minutes spent between swallows and yawns.) This normally non-patent state prevents the migration of bacteria-laden fluids from the posterior pharynx into the sterile middle ear.
Eustachian tube function is a complex affair; it's not easily tampered with. The following details what occurs normally with swallowing (please keep in mind that the pharyngeal end of the ET is above where a bolus of food or saliva passes during swallowing):
Normal ETs had four consistent sequential movements: (1) palatal elevation causing passive, then active, rotation of the medial cartilaginous lamina; (2) lateral excursion of the lateral pharyngeal wall; (3) dilation of the lumen, caused primarily by tensor veli palatini muscle movement beginning distally and inferiorly, then opening proximally and superiorly; and (4) opening of the tubal valve at the isthmus caused by dilator tubae muscle contraction.
A chronically patent ET is pathological (it's called a Patulous ET or PET), and is quite uncomfortable, so much so that ENT's try plugging the tube shut:
Trans-tympanic insertion of a new silicone plug seems to be useful for controlling the distressing symptoms of patients with a chronic patulous Eustachian tube (PET).
It should be obvious that on a purely physiological basis, a stented ET is not a good idea. In addition to migration of bacteria-laden fluids into the middle ear, there is a problem with sound conduction, inappropriate air movement with even minor activities such as whistling, making certain consonant sounds, the difficulty of something as simple as swimming, etc. God help the kid who would start laughing with a mouth full of partially masticated food (have you ever been so caught off guard by something funny while eating or drinking that it comes out the nose?) Imagine the mess that would make if the ET were continuously patent. It's a recipe for disaster.
Add to that the surgical complications, which would be considerably more substantial than with simple (yes, simple) myringotomy tube placement. Finally, those structures allowing ET function are delicate cartilagenous structures, and can easily be damaged by a foreign body (the reason even temporary, dissolvable ET stent use is not recommended). In the 80's, this was done in animal studies. Why you don't find papers of its use in humans is easy to deduce.**
Finally, you are mistaken that there are no attempts to treat (improve) ET dysfunction. There are surgical procedures available for those with severe ETD.
For millions of years, ear infections either resolved on their own, killed the host, or resulted in tympanic membrane (TM, eardrum) perforation. It was very, very common when I was first practicing to look at an adult's TM and see a healed perforation.
Yes, antibiotic resistance developed with overuse of antibiotics. But fewer cases of deafness, mastoiditis, and death from meningitis and brain abscesses occurred as well. We live and we learn.
**Not to mention that mucous can - and did in animal studies in the 80's - block the stent, alone rendering it more harmful than doing nothing at all.
THE EUSTACHIAN TUBE ABNORMAL PATENCY AND NORMAL PHYSIOLOGIC STATE
Analysis of Eustachian Tube Function by Video Endoscopy
Ballenger's Otorhinolaryngology: Head and Neck Surgery, Volume 1, John Jacob Ballenger, James Byron Snow, Eustacian Tube Dysfunction, pp. 201-208
The complications of chronic otitis media: report of 93 cases