1) There is a chance the deal is blocked by antitrust regulators, and does not close.

2) Regulatory reviews typically take 6 months to a year. There is an opportunity cost to holding shares over this time, your capital is tied up in shares, when you could instead use the money to buy US 10-year Treasuries that yield around 5%.

Assuming the deal closes at $65 in 12 months, the present value of these shares is $65 / (1 + 5%) = $61.90

I think the discount is the risk of the deal not closing; there will be regulatory approvals, financing, all sorts of offramps ahead.

Also, the market has consistently undervalued Instacart since they went public to begin with, so why stop now? (signed, a not-at-all-bitter former employee)

I don't know why for this specific deal but this could happen if the market expects that the deal won't close, for example if regulators blocked it