Because it's very dangerous and first responders access in national parks isn't always easy. You can obtain a permit to do it, however, see this memo that summarizes the current situation:

https://www.nps.gov/subjects/policy/upload/NPS_Guidance_Memo...

NIMBYists concentrate their efforts on national forests and parks, so everything is illegal there. Cross the boarder outside of Yosemite, and you're in unregulated forestry land where you can camp for weeks without a permit and walk wherever you want.

Most likely to avoid wasting emergency services.

Although, it would be nice if we could give people a general “I understand the risk and won’t ask for help if it goes wrong” waiver for dangerous activities.