Instagram won't let you do this on desktop because this is an intentional move to encourage you to use their mobile app so it forces you to engage with users. But this removes the ability to let you upload straight from your computer which could inconvenience many people who take photos from their camera or work with lightroom, photoshop, etc.
You may need to select a device on the drop down menu on the top of the page after you click on the mobile device icon and select a phone to load as, if it doesn't work.