Or, less charitably, they have provided a trove of documents to the buyer so the buyer can outgrow their business using a competitor.
https://m.youtube.com/watch?v=JlwwVuSUUfc
https://www.quora.com/Silicon-Valley-Season-2-Episode-2-Runa...
Older incumbent companies, especially, may have giant 'business development' teams who almost recreationally do deep x-rays of emerging threats/opportunities. All their staffing/trips/flirtatious-discussions/legally-drafted-non-binding-letters-of-intent may be a rounding error in their bottom line, a cheap research expense. They can go through all the motions of an acquisition, appearing serious to the hopeful founders, with a negligible interest in actually completing the deal.
I mean sure, they'd bite if they saw a can't-lose bonanza - their talks are panning for gold in your stream, before buying or even renting your land. Even if 99/100 envisioned deals eventually fall-through, they're just happy to learn all the proprietary business internals.
See also: ~pg's 'Don't Talk To Corp Dev': http://www.paulgraham.com/corpdev.html
"When a sufficiently high-up decision maker decides he/she wants to buy your startup, he/she will attempt to meet with you constantly and put time pressure on you, so as to prevent you from shopping the deal and getting a better offer. The absence of this behavior indicates the other company is not serious about acquiring your business."
One counter I would make to PG's essay is: investors, whether accelerators, VCS, or otherwise, predominantly benefit from big exits... and so they have that effect of pushing towards polarized outcomes ($0 or big). But "small" exits can still be very meaningful for founders.