The thing is, the guys that specialise in the ship-to-ship boarding actions, in space, are called marines... space marines. They're the elite 'special forces' who get te difficult missions - clearing Eldar pirates, taking Hulks, that kind of thing.
The IG is more like... 10 billion tiny speed bumps to slow the enemy down as it inexorably rolls across a planet, giving the Imperium's elite forces time to organise a counter attack.
It's been said that 'space marines take planets, guardsmen hold planets'. That really sums up the different roles they have.
"Well it's Forty-one Thousand Nine Hundred Sixty-nine OK -
Gotta war across the Milky Way - "
Iggius Popiscus and the Stoogii, "41,969"