There are two main factors leading to the strong increase in ETH price after the Merge:
The inflation rate caused by new ETH mined per year is determined by dividing yearly-mined ETH (4.93 million)/ total ETH supply (119.3 million) equalling 4.13%. After the Merge, the ETH blockchain will change from using PoW to PoS, and miners are forced to quit mining under the effect of the ETH difficulty bomb. This means that there will not be new ETH to be mined by miners, therefore, the above inflation rate of 4,13% will disappear.
Validators must lock 32 ETH into the Beacon chain to be accepted to validate transactions. Besides, the distributed rewards for validators are much less than the rewards paid for miners since validators are not subjected to intensive economic worries about mining machines and energy fees as miners.