Why SI System do not permit 10 to the power 4, 5, 7 etc.?

Answer 1

who told you that? the SI police?
ive never been fined for that, so im assuming its still acceptable in modern society to represent an order of magnitude of 10 to the power of 4, 5 ,7 or all three at once if i feel so inclined....

Answer 2

If you use multiples of 3, you can replace the 10^n by K or M or m or u or p etc.

50,000 meters = 50 * 10^3 meters = 50 Kmeters