You hear it from defensive players all the time; I've been playing the game this way since I was a kid. It's the only way I know how to play, etc. Troy Polamalu offered this up in a recent interview:

"I don't think any football player is going to go out there and change the way they're playing. I think it's too late in our lives to really do that. Of course, we're professional athletes and we try our best to adjust, but it's tough."
As a player on defense you grow up playing football in middle school, high school, college, and you're taught to get the player down at all costs. Dislodge the ball whenever possible and make your presence known to the offense. If you instill fear in them and get in their head they'll think twice about going up for that high pass across the middle knowing that you'll be coming to take their head off.

The players in the NFL now have been taught that type of defensive game their whole lives. They've been doing it this way since they came into the league, the old school way that their coaches taught them from decades of experience and knowledge. When players say, "I can't change who I am or how I play" there's a lot of truth to that. The best players in the NFL play on instinct and react to what unfolding in front of them. When you think about your game you lose a step, miss the tackle, miss the interception, or whatever. That's why you hear about young guys trying to learn the game so they can just play and not think about it. Today's NFL players that have been in the league for a number of years, especially the top flight players, all play on instinct. Sure they think about situations and how to best take care of their assignments but when the play is in motion they instinctively do what they know and what they've been taught over the years and learned from their coaches.

Quite honestly let's be realistic here, there aren't that many situations in a game every week that you get a chance or are put in a situation to really lay the wood down on a player. Sure there are a handful but the majority of the time you make your basic tackle and go on about your business. It's those handful of plays that get all the press hype and analysis from the league. That's what they want you to see, the big hit that makes James Harrison look like the bad guy. Never mind the 6-8 tackles he makes per game that are perfect form, completely legal, stops.

Basically what I'm getting at is that if the NFL wants to change their league to as little contact as possible, then it's going to take years and years to do so. Until the next generation of college players come out of school learning how to tackle the way the NFL wants them too, rather than the old school way.