High CPMs can lead to an inability to keep your ads profitable. Which is a big problem for any business trying to get leads or sales from paid advertising.
There are a few main reason why your CPM is high. The first is your Quality Score/ Relevance Score. Both Facebook and Google use these metrics to determine two things. The quality of the landing page you are bringing your customers to and how relevant the ad is to the people you targeted.
If either of these are lower than average your CPM will be higher than normal.
The second reason your CPM could be high is because of the people you are targeting and how your competition inflates the price. If you're targeting a broader audience of lets say 2 million there's a good chance your CPM will be lower than if you targeted a very specific group of 25,000 people.
There's only a limited amount of space either Google or Facebook can fit an ad in on their platform that your customer will see. If you limit the amount of people they can display the ad to and there are hundreds of other advertisers trying to get in front of that same small group of people the CPM cost will rise. There's just simply not enough space to fit everyone's ads.