Since Harry has a piece of the Dark Lord’s soul, why is that he can be injured so easily? I ask this question because I don't understand why the Horcruxes are so hard to damage, but Harry can get hurt like any other person can.
Harry wasn’t given the protection that a Horcrux is when created.
Horcruxes aren’t strong simply because they contain a soul fragment. The wizards who create them have to strengthen them using strong enchantments to keep them from being easy to destroy. It’s part of the process of intentionally concealing a piece of soul in an object.
“No,’ said Ron, before Harry could answer. ‘So does it say how to destroy Horcruxes in that book?”
“Yes,’ said Hermione, now turning the fragile pages as if examining rotting entrails, ‘because it warns Dark wizards how strong they have to make the enchantments on them. From all that I’ve read, what Harry did to Riddle’s diary was one of the few really foolproof ways of destroying a Horcrux.’
‘What, stabbing it with a Basilisk fang?’ asked Harry.”
- Harry Potter and the Deathly Hallows, Chapter 6 (The Ghoul in Pyjamas)
The Dark Lord never intended to leave a piece of his soul in Harry, so of course he’d never try to “strengthen” Harry like he would with his Horcruxes. In fact, strengthening Harry would work against his goals, as he believed that Harry Potter was the only person who could defeat him. He certainly wouldn’t, not even knowing Harry had a piece of his soul, try to make his mortal enemy stronger and more resistant to harm.