If someone were to write MIT licensed code that was heavily "inspired"[1] by reading GPL licensed code, people could then take that MIT licensed code and make proprietary clones and forks, effectively bypassing the GPL restrictions. It'd be essentially "code laundering".
[1] in a copyright infringing way, as defined by the courts in your preferred jurisdiction
[1] in a copyright infringing way, as defined by the courts in your preferred jurisdiction