Those processors are so cheap because they're old and power hungry and can only achieve the high frequencies when only a few cores are used. Once more than a few cores are used, the process will sit mostly at the lower frequency.
So your e7 2870 will be at 2.4ghz to 2.8 ghz and has a TDP of 130w, therefore two of those will use up to around 300 watts of power when you give them something to do. Compare that to modern processors that need 65 to 95w of power.
As a performance comparison, 2 x E7-2850 ( 2 x 10 core / 20 threads at 2.0Ghz .. 2.4 Ghz) in total have the same performance of a single Ryzen 5 1600 which needs only 65w to work. They're so old.
They were a good choice for companies which licensed software per physical CPU and where the license cost of the software was way more than the price of the cpu... the cpu was good if you needed those 20 threads.
As for why there aren't processors running at very high frequencies? That's mostly limits of physics... speed of light and speed of electricity. The speed of electrons flowing through "wires" is limited to slightly below the speed of light, which is almost 300,000k km/s or 186282 miles per second.
The higher the frequency, the smaller time electrons have to go from one place in a processor to another place, at speeds like 5 Ghz distances like half an inch are just too long. In order to reach very high frequencies processors have to be made in such ways that at any point, signals travel extremely small distance, and that's increasingly hard on modern processors that have a lot of transistors.
This is the reason cpu designers segment processors in individual cores running at reasonable frequencies like 3..4 Ghz because it's much easier to make small "islands" inside the processor, where at any point high speed signals only have to travel very small distances.
It would relatively easy to make processors like Intel Atom (very simple design inside, very small size) run at speeds like 8..10 Ghz , but that won't translate in super high performance ... a modern 6-8 core 3 Ghz processor will still run around it.