Well, there's two parts to it...
1. They'll need it later, and they can spend it now.
First, if you follow D&D 4e's treasure parcel system described on p126 of the Dungeon Master's Guide, they won't always have the best stuff. The rule books are actually fairly silent on the matter of how players buy items, but you can read more about that here: How do players acquire weapons in DnD 4e?
Basically, it comes down to...
And from my (at least rather limited) knowledge of the rules, they can't buy magic armor/weapons from the blacksmith in town.
They totally can, if you want them to be able to. There's a lot of options for how to handle this stuff: send them on a quest to meet the smith who can craft their items, let them work their way to a nearby city that'll sell them, or a myriad of other things.
Even if there's only small villages around and you want them to be able to buy magic stuff, a village's blacksmith might be holding onto one or two magic items nobody's actually come by to purchase (or could afford). You could ask your players to name a couple of items they want, and treat it as if those happen to be the two magic items the blacksmith happens to have for sale - how fortunate! (You can be totally open to your players about this too: tell them you'll treat certain villages as having a small quantity of magic items in stock that sets the limit for what they can buy.)
2. Finding incentive is actually their responsibility too.
What ways could I incentivize them to get out there and start cracking skulls?
Not all the responsibility here lies with you. Making the Tough Decisions is a must-read article on this note (in fact, it's must-read for players at my table): it asserts that your players have a responsibility to find a reason for their characters to accept the quests you provide them, so that they can keep the game moving and not let it grind to a halt.
A couple of particularly relevant paragraphs are the following (which, for context, follow a discussion of a problem related to Paladins):
Another useful application of this concept involves accepting story hooks your DM gives to you. Try to never just say, "My character isn't interested in that adventure." A lot of people mistake this for good roleplaying, because you are asserting your character's personality. Wrong. Good roleplaying should never bring the game to a screeching halt. One of your jobs as a player is to come up with a reason why your character would be interested in a plot. After all, your personality is entirely in your hands, not the DM's. Come up with a reason why the adventure (or the reward) might appeal to you, no matter how esoteric or roundabout the reasoning.
If the paladin is to blame for the last problem, this one belongs to the druid. Druids have such a specific set of principles that players often mistake them for being a free pass to demand that each adventure revolve around their goals. Raiding a dungeon for gold doesn't appeal to the druid mindset, so what are you to do if you play one and are presented with that goal? You improvise. Maybe the gold will enable you to purchase magic items that will let you protect the wilderness. Maybe the ruins contain unnatural monsters that need to be killed regardless of the treasure. Maybe, just maybe, the other PCs are your friends and you are willing to help them just because. Too often that last part is forgotten; I don't think anyone reading this has never spent the night doing something they'd rather not because a friend asked.
In short, if your players find their own reasons to accept the quest, you don't need to take it upon yourself to ensure they have radical or overwhelming incentive to accept quests. (You definitely should not do things like load them up with cash, unless you're clever and do it within the treasure parcel system.)
In fact, within this framework, there's the possibility of quests with virtually no material incentive at all: cue here Dark Souls, in which the entire game follows an epic quest of one Undead to break the Curse lingering over all Undead.