Why so few fares/bookable tickets with the DB Navigator site/app

Here's a concrete example, Frankfurt(M) Flughafen Regional to Bacharach on 23rd May.

Well, this example is an RMV ticket. Bookable with the RMV app or from the RMV website, or purchased from a Ticket vending machine.
https://www.rmv.de/c/en/tickets/purchase-tickets/rmv-app-rmv-handyticket-mobile-ticket

Got any other examples?

DB is not allowed to collect fares for regional tickets wholly within local transit authority districts, since that money has to go to the transit authorities.

As Sam points out , it is important to understand the distinction between DB ( German State Railways ) and Verkehrsverbund ( Local Transit Authority } In addition to the RMV site , here is the one for the Munich area , maps show the area covered - https://www.mvv-muenchen.de/en/index.html

Here's a bit of irony. Since Bacharach is at the limit of the RMV, you could buy a ticket from DB to the next station, Oberwesel, for 19.40 EUR and just get off at Bacharach. But you would be leaving money on the table, since the RMV fare to Bacharach is 12.70 EUR. However, if you were going to Oberwesel, you could buy an RMV ticket to Bacharach, plus a DB ticket from Bacharach to Oberwesel for 2.70 EUR. Now you don't get off the train at Bacharach, you just switch tickets, get off at Oberwesel, and you have paid 15.40 EUR. Take the 4 EUR you saved and buy yourself a beer.

DB is not allowed to collect fares for regional tickets wholly within local transit authority districts, since that money has to go to the transit authorities

But if the respective Verkehrverbund is cooperating, i.e. ready to pay a fee to the DB, one can buy verbund tickets via the DB app. Try, e.g., Nuremberg to Treuchtlingen. BTW, Omio etc. don't show more tickets, they just display connections which a long distance train is involved in.

Thanks folks. I was under the impression (from some YouTube channels, travel sites) that DB could be used as a one-stop place for planning and purchase. So I was expecting it to be like the excellent Swiss SBB app.

Do people think the forthcoming 9Euro pass will make the benefits of having a rail pass is much reduced?

I'm planning a 5 week trip to Germany, Austria, Italy In June&July and wondering if a 15 travel-day pass, or 2 month travel pass covering the three countries is going to be worth it for convenience, or if the 9 Euro pass when applicable in Germany is going to affect the value of a multi country pass lots.
(My rough list of places is no yet planned order is which will evolve and change during the journey is: Germany: arrival Frankfurt, Bacharach, possibly Cochen/Beilstein, Rothenburg, Berchtesgarden area, Garmish/Grainau/Fussen area, Austria: Salzburg and 3 or 4 Austrian lake spots east of there such as Wolfgangsee, Hallstatt, Admont, Traunsee; Alpbachtal area, Innsbruck and area; Italy: very numerous locations within the Dolomites, mostly in the area east of Bolzano, also Trento and bit of Trentino, fly from Verona back to Frankfurt, or instead of flying take time with trains back from Italy to Frankfurt).

Your judgement about DB is partially correct - When it comes to planning , it will pretty much show train routes in most places in Europe , so for initial roughing out a trip , it's a great resource . You can buy tickets for trips in Germany , and for some trips that either start or end in Germany. While it will show the routing in other countries , purchase won't work . One other tool for setting up rail trips is a map . Learning the geography of an area is helpful in figuring out advantageous routing . As an example , bring up a map of Switzerland and the immediate surrounding area ( Italy ) and read this post on the Switzerland thread https://community.ricksteves.com/travel-forum/switzerland/train-or-rental-car-d54991c2-efb7-4e40-9413-e62379461cb5 Here , Sam explains a better route , by train , for a trip from Switzerland to Lago Maggiore in Italy . Read his responses carefully . It's a perfect example of how to do this sort of thing . Keep the map on the screen as you follow his posts .

Unlikely a 15-day travel pass will save money. In fact, it would be a complete waste of money.

The 9 Euro ticket would work from FRA to Bacharach and to Cochem. To save 1 1/2 hours, you could buy a Supersparpreis ticket to Rothenburg for 42.90 EUR. Rothenburg to everywhere else in Bavaria you could use the 9 Euro ticket. All your destinations in Austria are in the same region so tickets will be inexpensive. Maybe a Sparscheine ticket from Admont to Innsbruck. In Trentino-Alto Adige, Italian train tickets are very cheap.