I think that's because it is taught by people who are not well trained in CS themselves for the most part, and a not so well defined curriculum. "You can build a cool website/mobile app" can help only so much in inculcating an interest in computing.

I wish there's something more like this book (https://en.wikipedia.org/wiki/Structure_and_Interpretation_o...) for schools which show that computation is a good extension of mathematical thinking (when used in the sciences), and beyond that, it can used to build "cool stuff" like mobile apps or software to control robotic arms.

In case my last sentence was not clear, I'm trying to allude to different roles of software. One can use it as a useful, very fast but dumb calculator doing computations to predict a molecular structure or a spreadsheet doing a bunch of stuff for an accountant/finance guy. Many times, we tend to think of it as useful to build cool tech apps/websites and force feed them Java/C++ or some such things which they're not ready for yet[1]. We should inculcate "computational thinking" before we teach them Javascript websites/java apps.

[1]As per my experience with some fancy schools teaching 5th/6th grade kids how to build their first apps