Simply put, because if you move them to an IRA - THEY ARE OUT OF THE PLAN AND NO LONGER A POTENTIAL PLAINTIFF against plan fiduciaries. One could argue that the only remaining liability for the plan fiduciaries is the decision to put the account with the selected IRA custodian - but the DOL provides guidance on how to do that - and once you've done that, the "statute of limitation" does begin to run.
Fees taken by the IRA custodian are NOT the responsibility of the plan fiduciaries. It is a classic case of, as a participant, "you snooze, you lose" BUT that isn't the plan's concern.
I have a problem with "forfeiting" subject to later restoration. Restore what? Only the principal forfeited? What about lost earnings? IF the missing participant is STILL a participant, does not the fiduciary have a continuing obligation to make the assets productive (trust law 101)? Can not the participant come back years later and demand that the account be restored with investment earnings? The interesting thing about "participant directed" 401(k) plans is that the trustee/fiduciaries are ONLY relieved of investment loss liability IF the participant ACTIVELY manages the account. If they don't, a solid argument can be made that the fiduciaries have an obligation under ERISA to manage the money prudently....