Wedding and event planner checking in. No, many people do not RSVP. They assume that you know how they would have responded, forgot to respond, or thought that no response meant they aren't coming. You have to contact them all. It's a pain. I know. I'm the one either making the calls myself or following up with the host to make sure they're made and get an updated guest count. Most venues or caterers need the guest count one to two weeks (sometimes more) ahead of time to make sure that their supply orders are submitted in time for delivery and preparation before the event.
Many people don't think about the cost of excess food/beverages/other items or headaches when you don't have enough food/beverages/other items, even though you planned for X number of people and Y showed up. We usually build in an extra 10% for non-RSVPers and last minute mind changers.
If more than 10% of people who didn't respond or who changed their minds and didn't let the host know show up, you probably won't have enough for everyone. That can include not only food and beverages, but ice, tables, chairs, glasses, plates, utensils, tablecloths, to name a few things that you might not think about. For a seated dinner, this might also mean you're short a server, which means people are waiting for their food longer than planned.
And if you change your mind (or something happens) and you just don't show up without letting the host know, the host is left with a lot of extra food and drinks on their hands, not to mention the $$$ they paid for the extra that needs to be donated or taken home by someone.
OP, your wedding planner is spot on! You got a good one. My best to the happy couple and their families. It's a very stressful time for all of you. Sending you lots and lots of hugs and pixie dust for a beautiful wedding!