Because they consume 2+ % more of your wattage, and that only gets worse the worse your chain maintenance gets! In addition, the idler produces more noise and potentially a rougher feel when pedaling, which can create a greater than 2% change in a subjective sense. Those losses are from chain link articulation on the highly tensioned top run of the chain:
https://www.pinkbike.com/news/tested-how-much-slower-are-idler-bikes-wh⦠If you find evidence that electronic suspension will provide 2% gains, then it could offset the idler losses, when compared to a non idler/non electronic suspension bike, but you would still be 2% behind a non idler...