yep, let me run and get closer to the shooter trying to tackle him so I can get shot at point blank range.
Maybe I can bite his ankle
Nobody is suggesting that you have to rush the shooter in hopes of making a tackle. Defense includes numerous options. You may try to hide, play dead, get behind cover, run away, attack with an improvised or formal weapon. Tackling an active shooter is just another option for defense. Depending on the circumstances, tackling the shooter may be one of the best options for defense.
Think about events like Luby's and VT. Not doing anything to stop the shooters meant that there was a good possibility that if you weren't already shot, the shooter may get around to finally shooting you. If already shot, not doing anything resulted in several folks getting shot additional times moments later as the shooter walked around shooting folks who were already shot.
Exept for a contact shot, your GSW likely won't be too terribly different whether you are 3 feet away or 30 yards away. Everyone shot in Arizona was effectively shot at point blank range. Most of the VT victims were shot at point blank range. The same is true for Luby's as well. You already may be at point blank range when a shooter opens up on people. If you think about it, point blank range is anywhere from the muzzle to somewhere between 20 and 40 yards for a lot of handguns. For rifles, point blank can be out to as much as a couple hundred yards. Point blank is going to be the distance as which the shooter can hit the target exactly where aimed or very close to it.
In several of these situations, people have felt that that stopping the shooter is critical, either as a matter of personal self defense (the shooter will eventually work his/her way around to shooting the would-be tackler) or in defense of others. Some responded as a matter of responsibility (such as one of the teachers protecting his students) and/or because they felt that they had an opportunity to make a difference.
Closing the distance to the shooter isn't free from risk. The closer you get, the less skill the shooter will need to land shots on you. The closer you get, the larger you appear in his/her sight picture. With that said, just about any method you use for self defense involves risk.
If you look at some of the examples I gave, most of the tacklers do not get shot while attempting to tackle the shooter, though several had been shot before making their attack. So it isn't a foregone conclusion that you will get shot while you are attempting to tackle the shooter.
Interestingly, once you get within 2-3 feet, your risk of being shot may actually decline as you get inside of his reach. This is especially true for shooters with long guns. When you get in between the shooter and the muzzle of his long gun, it makes it very difficult for the shooter to turn the gun around to be able to shoot you.