It’s not impossible, just inconvenient. Instagram was made to show off pictures, so when you open someone’s Instagram, all you see is a grid of pictures by default. If you want to read the captions and comment, you have to click on a pic and then click on the 💬 to view the comments and add your own. In a world where most places only make you click “send” to comment, it’s slightly more work than most people want for an online discussion.