Because Lexus had to justify the F-Sport package which added zero performance improvements, so they bolted on some shiny metal.
The 2017 RX (and even the latest RX) is about a decade behind in technology compared to Tesla & the German manufacturers. When you invest very little into R&D, you can afford to add shiny metal bits.
While I didn't spell this out, luxury sports models are just about the one exception to the rule. Lexus is a luxury brand and that is a sports model.
Most vehicles are not luxury or sports models so my statement is still accurate.
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EDIT: Looking up replacement parts, Toyota and Lexus uses a "stud" style pedal that's bolted into the accelerator at the floor. In this case a "cover" isn't really applicable.
In fact, I think you might have the only manufacturer that actually doesn't use a cover. Likely in response to their pedal scandal as older Toyota models use an arm with a cover.