because integration by substitution (sometimes) is an invalid argument for a lot functions that are to be integrated
for example if you try to integrate x^2 with the substitution u=x^2, it is not a valid argument, doing it by inspection and understanding the reverse-differentiation is a valid argument, faster, and is what separates you from a robot
and heres another example of why using integration by sub can result in disaster
try to integrate e^x^2 * cosx with the limits of (x, 0, 1), you'll see it cannot be done by substitution, or at all, really, but by INSPECTION we can use the trapezium rule on it, or explain how its an odd function, meaning it'll equal 0 every time, thus its a better argument to use inspection here
d/dx is confusing in that form --> d (a function here)/dx is much better, it means you are entering a function, such as d(2x)/dx which is then just 2, to be differentiated with respect to x, implying x is not a constant, this is important when it comes to understanding partial derivatives
edit: text books at a level and at the start of university tend to be dreadful, its a game of trial and error, picking authors that produce learning books that are suited to your style, try to look at some youtube videos on maths to help your learning on integration