From when I ate my first pickle up until now, I thought pickles were fermented. Turns out, only some are.
Most store-bought, big-brand pickles are not fermented. Instead, these cucumbers are just marinated and stored in a vinegar (and spices) brine. This method of pickling is called fresh-pack. These types of pickles don’t offer the same probiotic benefits that fermented pickles offer.

Notice the “fresh-pack” descriptor in red
What is the Difference between Pickling and Fermenting?
By definition, to pickle means to preserve in vinegar or brine. Note there is nothing about fermentation in this definition.
Of course, you can still pickle cucumbers and other things the traditional way by fermenting them (lacto-fermentation). When cucumbers are actually fermented, bacteria breaks down the sugars found in the flesh and turns them into lactic acid. This buildup of lactic acid is what gives fermented pickles their vinegar-like taste.
So are pickles fermented? Not all of them. However, things that are fermented and stored in their own lactic acid are also considered pickled like these tasty homemade fermented Brussels sprouts and these fermented jalapeños.

The intersection of pickled and fermented foods
The problem is (on a commercial scale), fermenting cucumbers takes longer than fresh-packing them. Extended production time leads to higher production costs, which I’m sure is a big reason most big-brand store-bought pickles are not fermented.
I highly recommend you make or buy yourself some fermented pickles. The flavors are on a different level. And unlike fresh-pack pickles, fermented pickles are good for your health.