Just to clarify, are you talking about a few opportunistic orcs turning up and running off with livestock from an outlying farm? Or are you talking about an organised raiding party/small army that gathers and moves towards Phandalin with the intent to invade and occupy it long enough to loot it, then cart their plunder back towards the mountains?
Are you also making the assumption that if a place with Waterdeep's resources decides to send a group of high-level adventurers to defend from or retaliate to a raiding party at an isolated town they are going to expect them to walk there?
Even if the response party arrives too late to actually defend the town, they can probably ensure that the raiders never make it back to their tribe deeper in the wilderness. Stories of how the tribe lost half of its members three generations back can be an effective deterrent.
Looking at the Lore I found on the FR Wiki, there is a major unifying Orc force called the Brotherhood of the Scarlet Scourge in the Sword Mountains.
At one point they drove out the Trolls, causing something called the Trollwars.
They have gathered four hordes at various points in history to attack and try and destroy Waterdeep.
Twice they were defeated in the attempt. Twice they were thwarted by the intervention of a Dragon. One was a Bronze, one was a Gold.
So, this is a force that has gathered multiple times to attack one of the strongest cities in the region, and twice needed to be stopped by a powerful Dragon. If it were actually possible for Waterdeep to hit them hard enough to be an effective deterrent, do you think they would have just not done so after the first horde tried to destroy the city?
And this is just one force in the Sword Mountains. This doesn't cover the threats in the Neverwinter Wood, the Mere of Deadmen (which is just bursting at the seams with threats, and while not likely to attack Phandalin, they are right next to Leilon, which would also need to be rescued by Neverwinter or Waterdeep) and The Kyrptgarden Forest.
Actually, I just pulled up Phandalin itself to see if there were any notable locations nearby I missed, and look what I found
"Phandalin was originally a farming community. However, when the orcs of the realm of Uruth Ukrypt had destroyed all game in their realm, they went in search of food and turned on the human settlements in the area, raiding them. Phandalin was one of these settlements, and in 951 DR, it was overrun and then abandoned."
That Uruth Ukrypt was one of the groups with the Scarlet Scourge, in fact it was the tribe of the founder of the organization. Which had already attacked and effectively destroyed Phandalin, seemingly with no repurcussions from the nearby major powers, since, looking at the timeline... yep, that was between the raids. The Orcs formed two more hordes in the years following that attack, and this was not too long after the gold dragon prevented their attack on Waterdeep.
So... yeah. "We are safe because they would fear reprisals" is not how the setting works, by the settings own lore.