If they’re truly “afraid of commitment” and not just horny, undecided and exploring their options, it’s because giving yourself entirely, caring for someone that much and finally changing your identity to being part of something bigger than just you can cause a lot of grief if something goes sideways (I mean, death is a promise and all but that’s more digestible compared to being broken up with, specially if it comes much later in life) and some people are very afraid of that potential result.