I'm a newer DM, and I'm running a game for five players of mixed experience. My two most experienced players both started in the last year or two, I've got two where they've played one or two sessions prior to my game, and my last player is new-new and this is his first experience.
Okay, so remember this - it is going to be important.
These are all my friends who I know from outside D&D. (One is actually my spouse) Our game is played largely as an excuse to hang out and spend time together. My players seem to enjoy the game, and they tell me as much, but part of me feels like they'd have just as much fun playing a board game or party games.
They wouldn't, at least, not for long - board and party games have limited replayability. There's only so many games of Pictionary you can play before it becomes old hat.
Part of me fears that the answer is going to be that I need to find another group who is as equally invested in the game as I am, and just accept this group as my chill hang out with friends group.
If you want it next week, yeah, you will. The phrase, "Patience, grasshopper," comes to mind. Rockstar players who understand that you can, and understand how, to engage and drive the action do not regularly leap fully formed from your forehead with a set of dice in one hand and a 20-page backstory in the other.
Long-term campaigns run for 18-months and more, right? So, even the most experienced of your players has played in maybe one? That's, by my measure, still a neophyte. And the least experienced among them is probably still trying to figure out what die to roll when. The whole bunch of them are still building the basics, but you are looking for advanced concepts.
Don't get me wrong - that is a glorious period in a gamer's career. They're still all, "Wait, the chest bit me? WTF is a 'mimic'!?!" and, "What do you mean? They can't all be trapped!..." They are still learning what is, and what's possible.
If I may be so bold, the best thing you can do for them, your job, as their GM, is not to try to make them into the Companions of the Lance just yet. Your job is to set them awash in the excitement of the newness of it all! Give them action! Danger! Surprise twists! Monsters! Traps! The thrill of victory, and the agony of defeat! If your resulting campaign kind of resembles a series of loosely connected pulp novels, you'll be doing them a favor. They're first outings won't be Pulitzer material.
I'd love to be able to work my PC's back stories into the plot of the game, but there are no back stories.
Yeah, so, the idea that the backstory matters, indeed that what happened last session matters, is a thing that takes time and experience to build, and internalize. Meanwhile, one of your players has yet to toss their first fireball.
You don't have backstories? That's fine. You're playing 5e? They all have backgrounds - lean into them. Make there be library adventure for the sages and scholars, and desperate commoners for the Folk Heroes, and so on.
Meanwhile, realize that they are building backstory with every session! In Session #2, what happened in Session #1 is backstory! Make it matter! NPCs come back for help or vengeance as appropriate, failures have consequences, victories have consequences! Choosing not to get involved has consequences! Develop allies, enemies, and rivals that come back! Clearing out the nest of rats means the town council thinks of them when the goblins come calling, which makes a name for them with the local Baronet when the organized bandits strike. Their success with the orcs makes the Baron wonder if the party is going to get in his way as he makes a move for the seat of the Count above him, so a spy is sent into their midst to take their measure - the spy falls in love with one of the PCs, despises another....
I think you get the picture.
They're perfectly content to just follow the "Main Plot".
At their stage, there is absolutely nothing wrong with that! Let them build up a history, and then give them clear, binary choices based on that history. When they are used to handling that, give them three options. Then four.
Then, you can start figuring out what kind of players they really are, because they don't have enough experience to know, yet. Not everyone is best served by an open sandbox of infinite options - option paralysis outright kills some people's creativity!
And, you mention that you, yourself, are still fairly new as a GM - you're going to be building skills too! Give yourself time and space to do that.