Why is there still a hole in the ozone layer?
To some extent, there most likely will always be an area over Antarctica with reduced stratospheric ozone content in late Southern Hemisphere winter. (I wrote "always". Read that as meaning for the next several millions of years.) The issues is how long that annual ozone hole exists, how large that ozone hole becomes, how deeply depleted that ozone hole is, and (most importantly) what that means for the world at large. The reason it will continue to exist to some extent is that volcanos can inject ozone-deleting chemicals directly into the stratosphere.
However, right now, the worst of the ozone-deleting chemicals in the stratosphere are chemicals created by humans. Many of those chemicals will remain in the atmosphere for decades after production of them is ceased. For example, the graph below depicts the amount of trichlorofluoromethane (aka Freon-11, and other names) in the atmosphere:
From the graph, it's apparent that the concentration peaked around 1993 and has been slowly declining since then. It has been almost 30 years since the production of trichlorofluoromethane has been banned, and yet the levels remain well above the late 1970s level. Ozone-depleting chemicals such as trichlorofluoromethane take decades to centuries to leave the atmosphere, even in the case that production has been completely eliminated. Trichlorofluoromethane, for example has an atmospheric half life of 40 to 100 years, depending on which article one reads. The Montreal Protocol does nothing to address the long-lived nature of those ozone-depleting substances. Once those substances get into the atmosphere, they are going remain in the atmosphere for a long time.
The Montreal Protocol did not tell countries to immediately stop producing ozone-depleting substances and to collect and destroy all known stockpiles of ozone-depleting substances. What it did do was to issue a phased shutdown of production of those ozone-depleting substances. Some such as halon were phased out more slowly than freon. Others such as carbon tetrachloride have not yet been completely phased out.
From the above graph, there was a slight stall in the decline starting around 2013 or so. This apparently was illegal production in Asia, later chased down to China. But this was a slight stall. The important thing to read from the graph is that it will take a long time for those human-created ozone-depleting chemicals to vanish.
To make matters worse, the replacements for substances such as Freon and Halon also tend to be ozone-depleting substances. They're not nearly as bad in terms of ozone depletion potential as Freon and Halon, but they still are ozone-depleting substances.