It happens because, by default, Ivy has horse sperm inside, instead of thermal paste.
It's a well-known fail of Ivy. The only two ways to fix it are:
1. Change horse sperm for a real thermal paste/liquid metal (usually people buy and use Nanoxia TF-1000, it's a liquid metal that helps significantly, but it's not cheap and it's not easy to get. Arctic Cooling MX-4 is much easier to find/get and it costs quite less, but of course, performs a little bit worse). The main problem in this method, is that you'll have to manually open the cap on your processor, and inexperienced people can easily make grave mistakes there.
2. Change Ivy for Sandy. Sandy has direct soldering, no horse sperm/thermal paste fillings at all (that's why it performs quite better than Ivy, at high clocks).
Basically, Ivy's horse sperm starts failing at 4.4~4.6 GHz and up, that's why below 4.4 it has decent temps, but at 4.5 and higher turns into a friggin' stove.
At frequencies lower than 4GHz Ivy is actually better than Sandy, but when it comes to high overclocking Ivy loses to Sandy in every aspect possible.
Of course, that goes only for default Ivy with default horse sperm inside, not in case if you change it to liquid metal or any other good thermal paste.