2^27 is 128 megabytes. How much RAM do you want the decompressor to have to allocate for every file? Especially since you can't tell, by looking only at the file size of a compressed file, how many bytes it will decompress to. You could read the file header, but if it's a malicious "zip bomb" type of file, the header could be lying.

If the header says the file is smaller than it really is, you've already allocated a small widow by the time you realize it lied, so it doesn't harm you here.

If the header says the file is bigger than it really is, it can get you to allocate a pointlessly large window. But if a large allocation is the goal, they can make the file actually decompress that big without affecting the compressed size. So lying is pointless.