I read a little into the subreddit's FAQ, but couldn't find an answer to this specific question: Why was there ever a reason to line up across the enemy and slug it out? Either your forces are better at firing muskets, in which case the enemy has no incentive to just stand there, or the other way round. The third possibility is if both forces are equally skilled. But then you would gamble on the outcome, since it can go either way. The only 3 scenarios i can imagine in which you just want to draw it out are:
Your enemy is stupid and you will win.
You want to delay the enemy, knowing you will lose in that specific engagement but win somewhere else.
You want to out-attrition your enemy, knowing you will lose now but win later.
If none of these cases apply, why engage in a drawn-out fire fight?
edit:formatting
Well yeah you got the idea. Just standing there and "slugging it out" was not ultra common; we have to remember this was true combined arms warfare. Infantry would almost perpetually be maneuvering and remaneuvering in response to enemy posturing of infantry and cavalry, artillery bombardments against their foes or on themselves, or being flat out attacked. Rarely did two sides just forget about the rest of the battle and get into a shooting match with another line; it was a giant cohesive effort.
That brings us to our main point then which you brought up; the principal of distraction. Apply pressure over a broad front, force all your enemies forces to commit to engagement, and then force your reserves into their weakest link and roll them up. That is the principal of Napoleons style of warfare.
You ask why would the enemy choose to fight back? Because the only other option is withdrawing from the battlefield. You could apply this to about any other era of warfare. Why would a smaller phalanx engage a larger one? It depends. Maybe they underestimated numbers, maybe they can't retreat, maybe they're holding off for reinforcements, maybe it wasn't as one sided as we see retrospectively. However most of the reason, until Napoleon, is precisely what you said: people didn't like actually engaging in the 1700's because it was fundamentally a gamble; it's called the era of indecisive warfare colloquially by historians for a reason.
The entire thought process of this period was how to avoid engaging the enemy and still subdue them; hell that is true for centuries before as well with a brief exception in the 30 Years War. Battles (and by extension sieges) were inherent gambles and thus fortress rings were constructed to ward off both and turn warfare into a game of who can out-raid and out-attrition the other in the long term. Ever wonder why wars lasted for decades on the past? That's the rough outline basic explanation why from a purely military strategy standpoint.
So they realized it was risky; Napoleon (and Frederick but that's another story) realized decisively engaging with combined arms and rapid bravado could win the day. Cavalry is not a tool to mop up it's a tool to commit the enemy reserves to the flanks. Your reserves aren't there to save a lost battle they're there to line up in columns and bayonet charge the enemies weak points once they're out of reserves. He didn't plug holes he made them with concentrated bayonets and grand batteries of artillery. The principal decisive weapon was the bayonet; it is far more useful in my view to think of pikes being replaced with bayonets that could shoot rather than pikes being replaced by guns with bayonets if that makes sense.
Any questions feel free. I'm at my parents house so no row of books in front of me but this all generalized knowledge acquired from Christopher Duffy's Fire and Stone: The Science of Fortress Warfare 1660-1860, H.W. Koch's The Rise of Modern Warfare: 1618-1815 and Russell Weigley's The Age of Battles: The Quest for Decisive Warfare from Breitenfeld to Waterloo.