Probably the same incentive you have right now not to omit something: if they do an audit, and find that you never told them about your self-employed second job, or whatever, you'd have to pay up.
Except if your return has no deviations from what the tax authority sent you, why would the computer flag it for an audit? You're essentially going to be in the randomly-selected pile, and the auditor won't see anything at all suspicious.
Besides that, unless you have income above $1M per year or so, it usually costs the government more to audit you than they will get from you as a result of the audit. So unless you're evading so egregiously that it makes sense to prosecute you criminally, pour encourager les autres, it's better to let little cheats slide. Like over-declaring the value of the clothes you dropped off at the thrift shop as charitable deduction. There's no way those jeans are worth $20 now, you cheater. But as long as your total charitable deductions are under $500, no one cares enough to nail you for it.
Most of the people who would sign and return the pre-filled forms probably haven't ever filed a schedule A, anyway, as it would turn out to be less than the standard deduction. Unless you really care about how much you pay in taxes, there is a huge incentive to just sign and return the form the instant you get it, without even looking at it.
