Its pretty simple to understand.
Pre November 2019, Google had 100's of schema types that could include Ratings Micro Data and if accepted, the search results would display Star Ratings on them.
THEN NOVEMBER 2019 HAPPENED.
They made a MASSIVE CHANGE that literally cut down 100's of schema types that they would accept Ratings Micro Data, to a list of only 17 schema types, all with specific requirements (no more "generic" ones).
These are the allowed schema types:
Addons like XFRM and Showcase are using the "CreativeWork" scheme type, which is a Generic "cover all types of content" type schema. The "CreativeWork" schema can (and is) still being used, however, it will no longer accept Rating Micro Data.
So if you want to use Showcase and get GOOGLE to ACCEPT RATINGS micro data, you would have to customize Showcase to use one (or more) of the above 17 Schema types. This is not something that you can just HACK a template and WALLA. Its going to require PROGRAMMING for each specific type, because if you attempt to CHEAT THE SYSTEM, Google will catch it and penalize you (and I am sure that is not something you want to have happen). Probably the one that would work for you would be PRODUCT, however, to implement that one, its going to require additional fields in the database that would store the data that is required by google. It would also require extensive modification to the existing application/ld+json output (which is what gets sent to google for their consideration for indexing).
You'll have the same issues with ANY addon that was using any of the Schema types that have since been removed (and most XF addons were using the generic catch all schema type "CreativeWork".
Mike (one of the Owners of XenForo) has already stated that they will NOT be making any changes to the core XenForo Resource Manager (because the amount of work is just way to extensive).
As mentioned above, I have plans to modify Showcase (and several other addons of mine that have ratings/reviews), however, that is a long ways down the road (probably sometime after XF 2.2).
btw, this only applies to Google. All other search engines accept ratings/reviews micro data from ALL scheme types still.
I hope that answers your question. You certainly don't need to have an entire ADDON developed for you when there are existing addons that only need some customization done to them.