As with most Google PPC products, the devil is in the detail. If it's a bottomless pit, your products are either not competitive on price, or more likely, your products and shopping campaigns are badly setup and managed.
Have you tried Performance Max campaigns instead? Google is slowly moving away from Shopping campaigns. We are using Performance Max campaigns for most of our clients and they perform very well.
It is possible to advertise only on shopping through performance max campaigns. If you don't add any assets in the performance max campaign then Google only shows your ad on shopping. However, in our experience the performance max works better if you don't restrict it only to shopping.
How are you looking at the reporting in terms of your campaign? I find that the results can offer an insight into what's not working or working. Perhaps look at refining your product feed to ensure it works with Google Shopping and other platforms on Performance Max. People can decide not to interact or click on ads based on many things. Are your images hi-res? Is the product descriptions effective? Is the price not competitive against other products their seeing?
I’ve just been using keyword ads, but my competitors have a bigger budget and use display, keyword, and shopping ads. They’ve stopped using shopping ads now, though—maybe it didn’t work well for them