> They couldn't allocate 5 more bits somewhere to let the decompressor autodetect longer window sizes?

It's actually 8 bits: https://www.rfc-editor.org/rfc/rfc8878.html#name-window-desc...

These command line parameters change the maximum the decompressor will allow. It's 128 MiB by default in the command line decompressor; other uses (like the "zstd" content coding for HTTP in web browsers) use a lower limit of 8 MiB (see https://www.rfc-editor.org/rfc/rfc9659.html).