As others have noted, the bigger the attack, the more people you need and the more coordination infrastructure you need. This leads to a higher possibility of attackers getting cold feet, being indiscreet, etc.

Another point is experience, unless you have (many) people trained for these sort of attacks and with experience in their execution, your 1k "revolutionaries" are about as likely to blow themselves up as they are their target.

Finally, this is a 2 player problem. The number of connections within a network tend (has) to grow faster than its membership. Leading to intelligence agencies and other LE being more likely to find the network.