Gaffer Tape said:

Ugh. Why are they rebooting this franchise? It's only been three years since the last one! That was the amount of time in between the second and third movie! Couldn't they at least give the old series a decade or so to get cold first? I mean, continuing the same film continuity with new actors would be bad enough, but THIS?!

It's a business decision on Sony's part. A fourth film was started with the old crew but it wasn't going well - Sony was sticking their fingers in it like they did with 3 - so Sam and the gang walked.

Sony's contract with Marvel stated that if a movie wasn't in production by a certain date, they loses the rights to the character, so Sony cooked up this "reboot" idea in order to hold on to the rights.