Why are nanoliposome encapsulated antibiotics still so unavailable despite being so effective?
Liposomes are the first nano-drug delivery systems that have been successfully translated into real-time clinical applications. So far, all liposomal drugs are designed for intravenous applications with control release. Most of them are designed to certain targets such as cancer cells. That's why most of liposomal drugs are used in chemotherapy (Ref.1):

The main fact is those liposomal drugs are used mostly in hospitals with control of appropriate medical personals. For example, all seven liposomal drugs so far in chemotherapy are Doxil®, Depocyt®, DaunoXome®, Mepact®, Marqibo®, Myocet®, and Onivyde® as listed in Ref.1 used in cancer patients (in- or out-patients, but under the supervision of their medical doctors). Morphine sulfate extended-release liposome injection, DepoDur™, and non-opioid, bupivacaine liposome (trade name Exparel®) are analgesics used in pain management, specially after postsurgical procedures (e.g., bupivacaine liposome is a postsurgical analgesic used in the management of postsurgical pain and It can also numb an area of the body to relieve pain during surgery or medical procedures). In addition, VYXEOS® is recently FDA approved  (First approved August 3, 2017) liposomal drug for the treatment of adults with newly-diagnosed therapy-related acute myeloid leukemia (t-AML) or AML with myelodysplasia-related changes (AML-MRC). VYXEOS®, a liposomal daunorubicin/Cytarabine, is a fixed-dose combination of liposomal bound cytarabine and daunorubicin that delivers the two medications in a 5:1 molar ratio. Further, it is the ONLY treatment recommended by National Comprehensive Cancer Network® (NCCN®) for induction in sAML patients $\ge \pu{60 years}$. Also, in addition to its use in chemotherapy for leukemia, DaunoXome®, a liposomal daunorubicin, is also FDA approved to treat AIDS related Kaposi's sarcoma.
Based on these facts, therefore, it is safe to say that liposomal drugs are in use without aware of general public. Regardless to say that those treatments are very expensive as you have guessed.
I think main drawback to not available liposomal antibiotics is their high vulnerability to acidic conditions, thus no oral applications available (almost all liposome broke open in the stomach). So far, only literature available for acid stable liposome is Ref.2 and 3. However, we have to stop the development in that direction due to the lack of funding. Therefore, do not believe what you are hearing. Liposomal applications and their oral applications are coming pretty soon (someone else may follow our direction). Nonetheless, now we have a vaccine for COVID-19 within a year, don't we?
References:

Upendra Bulbake, Sindhu Doppalapudi, Nagavendra Kommineni, Wahid Khan, &quot;Liposomal Formulations in Clinical Use: An Updated Review,&quot; Pharmaceutics 2017, 9(2), 12 (33 pages) (doi: 10.3390/pharmaceutics9020012)(PDF).
Adelphe M. Mfuh, Mathew P. D. Mahindaratne, Maritza V. Quintero, Frederick J. Lakner, Ande Bao, Beth A. Goins, William T. Phillips, George R. Negrete, &quot;Novel asparagine-derived lipid enhances distearoylphosphatidylcholine bilayer resistance to acidic conditions,&quot; Langmuir 2011, 27(8), 4447-4455 (DOI: https://doi.org/10.1021/la105085k).
George R. Negrete, Mathew P. D. Mahindaratne, Adelph M. Mfuh, Maritza V. Quintero, &quot;Compositions and Methods Related to Acid Stable Lipid Nanospheres,&quot; US20110268653A1 (https://patents.google.com/patent/US20110268653A1/en) (PDF).