If you're not sure why, here's a guess why!
If you just evaluate buys turn-by-turn, you'll never get that Herbalist. At the time in the game where you might have only $2-$3 to spend, you don't need the +Buy, so it feels like you should buy something that'll help you get your deck going and not that Herbalist which only gives you a measly +$1 and a buy you can't use yet. By the time you have lots to spend, it always feels so wasteful to spend, say, $6 on an herbalist when you can get a minion, or $8 on an herbalist when you could get a province.
Both of those feelings are traps, and will lead you to never get that herbalist. You should be wary of them. You have to plan ahead - either get the Herbalist before you need it, in anticipation that you will later, or overspend to get it.
There's a thread somewhere on this board titled "-Stef- buys Herbalist with $13" or something like that.
It's obviously a tough judgement call sometimes - if you get it too early you'll spend time just using it as a terminal copper, which could hold you back, or if you get it too late you'll only get to use it's buy once or something. Ideally you want to get it just in time - the shuffle before your deck takes off and starts consistently generating $7p, or on the very first turn you're starting to consistently get big spending.