Because the guarantees attach to the bond, not the fund. According to TreasuryDirect, TIPS are issued with 5-, 10- or 30-year terms, pay interest every six months, and at maturity pay “either the increased (inflation-adjusted) price or the original principal, whichever is greater.” If you hold a TIPS to maturity, you know the real value you will get back. A fund keeps selling bonds as they approach maturity and buying new ones, so it never reaches that date, and its price on any given day depends on real yields that day.
Taxes differ by account too. TreasuryDirect notes that federal tax is due each year on the interest, that increases in principal may affect your federal taxes, and that there is no state or local tax on TIPS you own directly; with a fund, most states let you exclude the share that came from Treasuries (check the fund’s year-end tax notice). A fund distributes that income to you each year, so in a taxable account you can owe tax on inflation adjustments you did not spend. Many investors hold TIPS funds in an IRA or 401(k) for that reason.