So we're in a stable orbit with a high periapsis (closest part of our orbit to earth). To de-orbit we need to lower the periapsis so that it touches the atmosphere, and on each pass the satellite will slow down, further lowering the apoapsis and periapsis. The problem is that to do this we have to burn all that horizontal velocity that's keeping us in orbit in the first place, which is a crapload.

An alternative is to instead change the orbit so it's no longer in geostationary orbit and potentially interfering with live satellites, so instead they just push the satellite further out into the "graveyard orbit".

There's 100x difference in the needed energy for the two methods, and I'm sure you can see why most would opt for the graveyard orbit.

EDIT: To add, it's actually rather difficult to "plunge into earth". To do this you need to burn all of your horizontal velocity (I think that's around 3.07 km/s in geostationary orbit?). Reaching the sun is actually really difficult as well for the same reason, you have to lose all of your horizontal velocity, and earth has a ton.

Yes. you can accelerate away from the Earth and have a lower periapsis (conversely, you can accelerate towards the Earth and have a higher apoapsis). The only way you can maintain the same periapsis is by accelerating in the same direction you're going. Highly impossible in an explosion.

Also note, it is impossible to raise periapsis in a single event (or single time frame), except by leaving Earth orbit entirely (i.e. eject to planetary space)

I HIGHLY recommend both of them.

   Additionally, it takes a lot of fuel to be able to de orbit high altitude satellites.

De-orbiting to hit the atmosphere at a shallow angle would actually take slightly less delta-v than de-orbiting to a straight plunge to Earth. But both are much larger than the delta-v required to get to a graveyard orbit.

For physical intuition, uh, the atmosphere is a hundred times further away than the graveyard orbit.