A heavy tip does actually help with penetration vs a moving opponent.
If you hit someone with a spear, it doesn't automatically go right through them. You have to push it through with some force or momentum. It also rarely hits perfectly square. If you hit perfectly square, it matters little if the tip is heavy or not, you can just commit to the attack, and drive the attack home yourself.
We fight in a non-perfect world. No opponent stands there and gets hit square in the chest. They're dodging and weaving. Some of this dodging can be done while they are touching the spear. It may happen when the speartip strikes them, or it may be that they reached out to deflect the spear.
When they seek to deflect your spear, they have a simple goal. They need to change the line of motion of the spear such that it no longer goes through their body before the spear finishes its job. This can be done by moving the spear or by moving their body. Generally speaking, the spear is lighter weight, so it is typical to want to deflect the spear.
When they are deflecting the spear, you really can't do anything about it. You are too far away, with a long stick giving you horrible mechanical advantage. A skilled individual may be able to do clever tricks to overcome this, but for most people, the deflection of the spear is a 1 person game between the defender and the spear.
If the speartip is light weight, it takes little force to move it. If the speartip is heavy, it takes much more force to move it to a different line. This can be the difference between a hit and a miss.
At a more advanced level, a heavy speartip offers some interesting gambits. Because the spear will keep moving on its own after you stop pushing it (from its inertia), one may be able to stop pushing the strike a little sooner, and start claiming control of the stick earlier while the momentum of the spear continues to move it forward. However, this creates situations where the spear can overextend you, so those are more difficult to explore.
When exploring such advanced weapons, the momentum games are typically not analyzed to find the ideal balance. Instead, the weapons are used and fine tuned through experimentation and warfare. Whatever balance suits the combat style best is the one that is used.