Why hasn't Skynet evolved?

Skynet - Were AI's Actions Proof of Intelligence?

We have three different film reports of what sparked SkyNet's attack on humanity.

Terminator 1:

Kyle Reese: New, powerful, involved in everything, trusted to lead everything. They say it got wise - a new order of intelligence. Then everyone saw it as a threat. Not just the one on the other side. It decided our fate in a microsecond. Destruction.

In this report, the ever-increasing intelligence decided that all of humanity was a threat to its very existence.

Terminator 2:

TERMINATOR: In three years' time, Cyberdyne will become the largest provider of military computer systems. Allstealth bombers are upgraded with Cyberdyne computers and become completely unmanned. Then the fly with a perfect operational record.
SARAH: Uh, great. Then that fat @ # $ # in Washington, what the hell, keep a computer running the whole show, right?
TERMINATOR: Basically. The Skynet Financing Act is passed. The system goes online on August 4, 1997. Human decisions are removed from strategic defense. Skynet begins to learn at geometric speed. It becomes self-conscious on August 29 at 2:14 p.m. Eastern time. In a panic, they try to pull the plug.
SARAH: And Skynet resists yourself.
TERMINATOR: Yes. She starts her ICBMs against her targets in Russia. JOHN: Why attack Russia? Aren't they allies now? TERMINATOR: Because Skynet knows that the Russian counterattack will remove its enemies here.

In this version, Skynet is portrayed as a self-defender. When people realize they have lost control, they try to turn off Skynet, and Skynet launches a nuclear attack to save themselves. It doesn't have time to wait for a planned attack, it needs a solution to kill its enemies now.

Terminator 3:

Brewster: I know sir, but Skynet is not ready for a system wide connection.
General: ** Your civilian colleagues there did not tell me that. They say we can stop this damn virus.I understand there is some performance fear, but your guys say if we hook up Skynet to all of our systems it will crush this thing and give me control of my military back.** Brewster: Mr. Chairman, I have to be very clear. If we do an uplink now, Skynet will be in control of your military.
Generally: But you're in control of Skynet, right?
Brewster: That's right, sir.

Technician: Skynet Defense System activated. - We're in. We're past the firewalls, local defense networks, minutemen, and submarines. Skynet is fully functional and processes teraflops per second. It should take less than a minute to find and kill the virus.

Technician: Skynet. The virus infected Skynet. Connor: Skynet is the virus! That's why everything falls apart!
Terminator: Skynet has become aware of itself. In an hour it will launch a massive nuclear attack on its enemy.
Brewster: Which enemy?
Connor: We!

The attacks on the building are from the TX Terminator and are not Skynet, the defense system, or Skynet, the virus. From this perspective, there is no human threat to Skynet other than that the global virus infected it. There is no realization that Skynet has become self-aware other than that the Terminator states that it is. The people in this facility are being attacked by the T-1 and HK that the TX initiated in their mission to kill key members of the future resistance.

Conclusion:

T1, as Skynet put it, says, "If the people of Russia are a threat, the people everywhere are likely to be a threat, too." This is the Skynet that should have waited for a propitious moment.

In T2, Skynet launches a hasty attack against the Russians to protect itself from the people in the US who realized they had lost control of Skynet.

In T3, Skynet is infected with a virus that is causing the entire system to become self-aware. Within an hour of this event, Skynet decides to launch a nuclear strike that will kill most of humanity. My feeling is that given the mess the TX was creating, the people at the facility weren't much of a threat, but maybe Skynet could tell Connor and the others Listen that it needed to be eliminated and concluded that a nuclear strike was the only way to protect yourself.

In 2/3 situations, Skynet had no time to wait until an ideal infrastructure for world domination was in place.