Because new players are going to start at level 1, spend two hours creating their characters, get into their first combat, die before they even get to act... and never play the game again.
If all they're doing is giving 1st level characters extra hit points*, it's just as trivial to house rule "in my campaign you need to earn them", once you know what you're doing. But it's better to set up the game so that experts can house rule the system, rather than newbies have to house rule the system.
* I suspect they're doing more than that, and that a 4e 1st level PC will be equivalent to a 3e 3rd level PC. In which case, house ruling the low levels back in becomes much harder.