Because the IRS expressly tells you NOT to do that.
Here you go:
"For federal income tax purposes, a single-member LLC classified as a disregarded entity generally must use the owner's social security number (SSN) or employer identification number (EIN) for all information returns and reporting related to income tax. For example, if a disregarded entity LLC that is owned by an individual is required to provide a Form W-9, Request for Taxpayer Identification Number (TIN) and Certification, the W-9 should provide the owner’s SSN or EIN, not the LLC’s EIN."
https://www.irs.gov/businesses/small-businesses-self-employed/single-member-limited-liability-compan...
(When it says "the owner's EIN, it is talking about a single-member LLC owned by another company. If it is a person, the term "owner's social security number" is the applicable one)
