The US tax system is a "pay as you go" one. If you underpay throughout the year, you can be subject to interest and penalties. When you sign your W-4, you say [u]nder penalties of perjury, I declare that this certificate, to the best of my knowledge and belief, is true, correct, and complete.
2021 Form W-4 (irs.gov)
Further, if the IRS determines that an employee does not have enough withholding, it will notify the employer to increase the amount of withholding tax by issuing a "lock-in" letter that specifies the withholding arrangement permitted for the employee.
