Single inventors and small start-ups often do not investigate the full cost of patent ownership and exploitation. They begin filing a patent and then decide that they cannot afford the costs of continuing the process and seek to sell the invention during the patent application stage.
Typically the market is not open to acquiring single patents at an early application stage, so the inventor faces the option of having a very limited patent coverage to keep costs down with the consequential limited market, abandoning the patent application or seeking third party investment with consequential encumbrance from the third party.

Patents following the standard processing route can take three to five years to reach the stage of a fully granted patent. Inventors often do not realise that they might have to hold an application for that length of time before they can consider selling the patent

Credit: https://minutehack.com/guides/some-surprisingly-interesting-facts-about-intellectual-property