It's mostly the velocity. Orbiting a planet just means having enough kinetic energy to fall toward a planet, but continually overshoot and miss. They need some way to slow down dramatically, which is usually accomplished with rocket thrusters. If they just strapped on parachutes and jumped out, they'd orbit the Earth alongside the ISS.

ISS is around 400km of altitude. There is still a little bit of atmospheric friction there so you’ll definitely end up on earth but it will take years and you’ll probably burn while passing the Kármán line at 100km.

The good thing is that you’ll feel nothing at that moment cause you’ll be dead for a while due to no oxygen reserves.

The only way to escape is what they do today; a re entry pod with thrusters to do an entry burn at a scheduled time (if you burn at the wrong time and place you can go "higher" towards open space instead of back to earth but you also want to aim at your landing place and not end up in the middle of manhattan or a minefield), a heat shield for passing the atmosphere and then parachutes to soften the landing.

If you are curious about rocket science and like games, I suggest you to buy Kerbal Space Program 1, there’s tons of mods and you’ll learn while having fun.

An escape pod would need some thrusters, a heat shield, and parachute at bare minimum. Thrusters to cause reentry, heat shield to bleed off most of the energy while zipping through the upper atmosphere, and parachute once sufficiently slow and low enough.

Atmospheric drag would probably light you up. Maybe someone more versed in physics could go deeper on this.

The others already gave the reasons, but let me put some numbers to those answers. When I jump out of a plane, I get about a minute of freefall. That's because terminal velocity for skydivers usually is ~115-135mph, or 200ft/s. That speed comes when you're flying on your belly, putting about as much drag as possible in the wind. If you're in other positions, like head down or feet down, you can get going much faster (record for head down is over 300mph).

At ~120mph, the jolt from pulling a parachute is pretty good. We use specially designed rigs to actually slow the parachute's opening, so the jolt isn't terrible. When the designs don't work, it can cause injury or even death. That head down record I mentioned earlier? If you pulled at that speed, you are almost guaranteed to suffer major injury. It's why we recommend that skydivers DO NOT learn to do so until they are pretty experienced. There are even more specialized rigs to fly in those positions that ensure accidental deployments of the parachute are next to impossible while at those speeds.

Now, to have a stable orbit in space, relative velocity is vastly larger than even in freefall. If I remember, it's closer to 20,000mph (I think the ISS crew experience a 'sunrise' every 75 minutes). In order to get anywhere close to the safe parachute-pulling speed of ~120mph, you'll need to slow down a lot. Unfortunately, doing so is going to create (transfer if we're being physics specific) a great deal of energy in the form of heat, which will far exceed what they can safely handle. Don't forget that 2003's shuttle accident was because of damage to the heat shield of the shuttle, compromising its ability to keep heat from the crew compartment.