Because if you turn in a "mutilated" or damaged passport for replacement, the US Govt says that it will not give you that passport back, i.e. you would lose the visas included in it.

From their instructions:

To replace a passport that has a Material Alteration/Mutilation, you will need to apply in person with a DS-11 form, submitting:

* proof of citizenship (other than the damaged passport)
* acceptable photo ID
* passport photos
* all fees
* the damaged passport for destruction
* a signed statement (from the customer) explaining how the passport became damaged

For the location of the closest acceptance facility or passport agency to you, to download a passport form, or for the addresses of the U.S. State Vital Records offices, you may access our web-site at www.travel.state.gov/passport_services.html or call our toll free number at 1-877-487-2778. TDD/TTY 1-888 874-7793

Completed applications are normally processed within six weeks. If you need your passport sooner, you may want to consider expediting your application. The additional fee for expedited service is $60 per application.

For more information please visit our web-site at www.travel.state.gov/passport_services.html or call our toll free number at 1-877-487-2778. TDD/TTY 1-888 874-7793

Thank you.033
National Passport Information Center