Constant combat

Another thing to consider is how you approach encounters with respect to the reward system. Do your players receive any benefits to resolving encounters peacefully other than not expending their resources (HP, spell slots, consumable and limited-use items, etc.)?

Consider this: your players might not want to avoid combat because they don't get anything (or receive reduced rewards) when they do avoid combat.

This comes back to the terminology I use: encounter. An "encounter" is when your PCs are in the physical presence of and have a potential for combat with creatures that are in direct opposition to the group's objectives. These objectives can be as simple as "a bear wants to eat one of the party," and "the party member does not want to be eaten," or mundane as "the party wants into town to rest at the inn for the night" and "the town guard are under strict orders that no one enters or leaves town without the lord's permission," or as complex as "the party is responsible for escorting an allied NPC safely through a dwarven mine" and "the dwarves want to bring justice to the NPC because they believe the NPC is a thief."

There are two basic ways a party can deal with an encounter: avoidance or confrontation. Avoidance comes in many flavors: perhaps they're using stealth to get past, or maybe they're just turning around and running away. Confrontation, likewise, also comes in many flavors: sure, they can do battle, or perhaps they can engage in negotiation or deception, set traps--there are many methods that players can use to deal directly with the encounter. Ultimately, though, whether they choose avoidance or confrontation to deal with the encounter, reward them if their methods achieve their objective in overcoming the encounter.

Let's use an example inspired by DM Onesie Knight to show you what I mean.

The party has encountered a group of guards at the gate. Their objective is to get to the other side of the gate without raising the alarm, and the guard encounter stands between the party and their objective. Regardless of the methods they use to achieve their objective, reward them the full experience value of the encounter as long as they have gotten past the guards and the guards have not raised the alarm. Now, this scenario could work the same whether you're looking at town guards in a city under martial law, goblin guards at a hobgoblin fortress, or the lookouts at a bandit camp. And the methods the players use may vary depending on the situation: they're more likely to try to negotiate with the town guards or the bandit lookouts than with the goblins, and they're more likely to just try to fight their way through the bandits or the goblins than the town guard. Sneaking into a town or a bandit camp might be easier than sneaking into a fortress.

Once you adjust your thinking to rewarding them for dealing with the problem, not how they deal with the problem, it makes the next step easier.

Consider point two: how comfortable are your players dealing with their encounters with something other than combat? As Maslow's Hammer states, "I suppose it is tempting, if the only tool you have is a hammer, to treat everything as if it were a nail." The players know how to do combat. They are comfortable with it. Remind them that they have other tools in their toolbox, and that the reward will be the same regardless of the tool they use. If you've already been doing that behind the scenes, perhaps it's best to let them know in the open that is what you're doing.

Now, if you're going to let them do the alternate method for full XP, don't let it hinge on a single die roll. If a player tries to say "I convince them to let us through" and then rolls a natural 20 on the Persuasion check, that's hardly the equivalent of a combat's worth of experience. Most combats worth any experience don't last for only one decision, and one die roll. Make sure the party knows they're going to have to actually engage the encounter, however they want to do it. Then, when you run the encounter, narrate through the actions of the creatures they're engaging. Is it a negotiation? A chase scene? Stealth maneuvers? Walk them through each action. These encounters should take around as much time at the table whether it's combat or not if you're going to give them XP for it.

Now, last point to consider: is this what the players want? Some players live for "the talky sessions," as a DM friend of mine put it, and some players just want a beer-and-pretzels, kick-in-the-door, no-stress, no-consequences style of play. Most players are somewhere in between. Maybe your players haven't figured that ratio out for themselves yet. Let them know that non-combat resolutions can give them rewards, remind them every so often if it seems they forget, but if they still choose combat over all other options, maybe that's just how they want to play. But if your DMing style doesn't mesh with the kind of game your players want to play, well, that's another discussion entirely.