let's consider two movements: a deadlift and a prone hamstring curl.

the deadlift is something you've done a million times whether you know it or not, though the novice is probably more uncoordinated than they realize at higher weights. like it would be a struggle if not dangerous to move a couch, for example. this is a very practical skill to develop, and you're still "targeting" the same muscle group as...

...a prone hamstring curl. how often do you find yourself prone and flexing your knees against a load? probably never, right? how do you contextualize a movement like this to someone's day to day? what i mean is, what is even compelling about a movement like this and what does it actually mean to someone just starting out?

i'm the world's biggest proponent of "keep it simple, stupid" when it comes to working out, especially as a novice. super detailed split routines are more often than not overwhelming and set people up to develop bad habits while never meaningfully addressing the core issue that drive many people to go to the gym in the first place: people suck at moving.

"Targeted" exercises tend to be body-builder type stuff which can only really be done with light weights. Instead, bigger compound movements like squat and deadlift can be incrementally trained leading up to much, much heavier weights and they train the body as an integrated whole.

In daily life, the body is used to move with many muscle groups working together. By using a targeted exercise, you can create muscle imbalances and injury is more likely (especially stuff like tendonitis).

Lack of time is one of the biggest reasons people stop lifting. Standard full body programs are going to be more efficient time wise than picking muscle groups. For example, a program with a lot of rows and presses needs little to no biceps or tricep work.

It's not actually bad, it's just not the current Internet fad. If you like to work out that way, you should do it.