This is not the reason. The reason is that Relevance is strictly ReadOnly. They cannot allow arbitrary scripts to be executed while also enforcing Relevance’s ReadOnly status.
They do care about the resource usage of the client in general, but that is not the primary concern when it comes to arbitrary execution.
Relevance inspectors use various APIs, System Calls, and other things to read info from the system. It is not quite the same as execution. The client has code that it runs to power inspectors, but it isn’t executing external scripts to do so generally.
The run a fixlet periodically and read the results with an analysis is basically a workaround for missing inspectors.
You are really asking for a new inspector to be created on the platforms where it is missing. This new inspector could potentially use the command line tools to get this info, but it would much more likely be implemented directly in the code using some sort of API.
Please file an RFE for this with enough details, but also include a link to this forum post in the RFE: How to ask for IBM product help: PMRs, RFEs, and more