This is largely due to the fact that "Reels" is an add-on feature to the rest of the Instagram app, rather than its primary focus like it is on TikTok.
TikTok was built around this flow. Its community culture expects the content to be in a certain format and will self-moderate via metrics such as likes, comments, views. The UX of the app itself is built around this content. There are no distractions.
Instagram on the other hand has a very different community, with different culture and expectations. The content creators on Instagram may not be as familiar with the content that is conducive to the Reels format. The UX of Reels on insta is also an add-on feature. You're not opening instagram to interact with Reels, you have different intentions. Whereas when you open TikTok, you are there for one reason, so the app is built around that one flow.