Three reasons, and all three are install-quality problems rather than material problems: foam that was mixed off-ratio and never fully cured, foam sprayed over wet or already-rotting sheathing, and removal demanded by a UK-style lender process that does not apply here.
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Removal is genuinely expensive and unpleasant. Cured foam is mechanically bonded to the substrate and comes off by scraping and grinding, which means dust, disposal, and often replacing the sheathing underneath. It is worth understanding what actually drives it, because the causes are avoidable and none of them are inherent to the product. Off-ratio mixing. Spray foam is two components combined at the gun in a precise ratio and at a controlled temperature. Get the ratio or the temperature wrong and the chemistry does not complete. The result is foam that stays tacky, or crumbles, or emits a persistent fishy amine odor that does not fade with ventilation because it is not off-gassing, it is unreacted material. Ventilation does not cure this and neither does time. Removal is the only remedy, which is why it is the leading cause. Spraying over a wet or failing substrate. Foam is a vapor retarder in the closed-cell case, so anything trapped under it stays under it. Sheathing that was damp, or already beginning to decay from an unrepaired leak, gets sealed in. The UK lending situation. Homes there have been stripped to satisfy a surveyor or a lender. That process does not exist in the US mortgage market. The four questions that prevent all of this, asked before signing. Is the proportioner monitored and are ratio and temperature logged. What is the substrate temperature on the scheduled day and what happens if it is out of range. What pass thickness does the product data sheet specify and will the crew hold to it. Has the roof been confirmed sound and the deck confirmed dry. A local note: the shoulder seasons in Greater Columbus and Central Ohio produce mornings where an unconditioned attic sits below the product's substrate range and afternoons where it is comfortably inside. That is a real scheduling constraint here, and a crew that raises it before you do is telling you something good about how they work.