It's just because it...kind of just is pronounced that way. English is a mess of inconsistency.

If you start digging at one weird or contradictory spelling or pronunciation you'll never stop.

I find that the majority of native speakers almost use the language kind of instinctively?...without really understanding the 'rules' as such.

I'm sure someone cleverer than me will be able to explain the rules surrounding that particular pronunciation.

Just be aware that most of the 'rules' contradict themselves frequently.

🤷🏻 English 🤷🏻

Edit: Add to this the broad range of English dialects and accents (even just across England), many of us pronounce the same words quite differently.