Because mathematically you need to know the prior block hash before you can compute the next one.

And unless you have > 50% of the total network hash rate, then you can't permanently compute the next hash faster than the network can.

Note you can with declining probability P (< 1) compute the next hash faster than the network with less than 50% of the hash rate. Since probability is multiplicative, then doing this for two consecutive blocks is P x P, e.g. 0.1 x 0.1 = 0.01 so from 1 in 10 for one block rises to 1 in 100 chance for two blocks. Ditto for N consecutive blocks, e.g. for four blocks then 0.1 x 0.1 x 0.1 x 0.1 = 1 in 10,000 chance.