The same reason why i7-8700K is hotter than i5-8600K. Both are 6 cores, i7 have HT.
I built a 9900K for a bro here, using be quiet DRP4 dual tower cooler. This cooler is a Noctua NH-D15 equivalent. At stock settings, cinebench is at 80s, peaking at 90s deg C on sg ambience of 28-30 deg C. Without air cond ambience, it's almost impossible to consider. On 5ghz all cores, I can't run any AVX workload without throttling (yes hitting over 100 deg C). Cinebench at high 80s and mid 90s scatter around all 16 logical cores, some peaking over 100 and throttle.
But I keep the OC settings nonetheless as the pc is intended to be used in air conditioned environment, which load temps are lowered to slightly more feasible mid 80s on high load scenario. Otherwise, gaming is at 50s and 60s.
Keeping in mind, the build is inside a very well ventilated FD Meshify C, with 6x GT fans, all on custom fan curves to prevent turbulence and optimal acoustics when it's below 60 deg C. Once exceed, corresponding fans will ramp very aggressively to keep everything cool and under control.
Sent from OnePlus ONEPLUS A3003 using GAGT