Generally speaking, in mathematics linear problems are fairly simple to solve but multivariable problems are much more complex. The same is true for life.
Many problems in life are somewhat single variable problems where the outcomes is highly dependent on one variable.
Multivariable problems or systems problems as their also known as are complex problems with complex solutions. Alex Danco explains why systems problems are difficult to solve:
System problems cannot be fixed in one step, nor can they be fixed in a sequence of linear steps. Why not? Because when systems find a steady state — which is probably where you’re encountering them, if you’re setting out to change something — they’re “steady” not because they’re static, but because they’re dynamically held in place by feedback loops. If you try to change one variable, you can apply as much effort as you like, but the minute you let go, the system will just snap right back to its original configuration. - World Building, Alex Danco
Starting a business is a multivariable problem. Organizing a political campaign is a multivariable problem. Directing a movie is a multivariable problem.
Many of the hard problems that worth working on are multivariable problems.