Almost certainly because you're running too fast. It's the single most common beginner mistake, and it's not a fitness problem — it's a pacing one.
When you start out, your instinct is to run at what feels like running. But that pace is usually way above what your body can supply oxygen for, so you tip into breathlessness within a minute or two. The fix feels counterintuitive: slow down to a pace that almost feels too easy. If you can't say a full sentence out loud while running, you're going too fast.
This is fixable in days, not months. Most people are stunned at how much further they can go once they stop sprinting their easy runs.