(a) Because 75% of the Internet doesn't allow spoofing of source addresses,
and (b) Although there's a chance that one machine throwing 3,000 SYN
packets a second will show up on somebody's network monitor, you're never
going to see 3,000 network monitors pop on 1 SYN packet per second.
And oh yeah, (c) sometimes you don't want to spoof the connection but
want to actually *make* the connection, in order to send them stuff that
will consume even more system resources than just a dangling half-open
connection....