Why carrier is locked for my iPhone?

Carrier locking is not done by Apple. It's done by your carrier, to prevent you changing networks after getting the phone. Certain carriers offer unlocking- you can check the list for your carrier here.

For instructions on getting it unlocked, see here - How to unlock your iPhone for use with a different carrier - Apple Support

If by "raise their hand" you mean "shift responsibility", then no. Carrier locking is generally done to prevent people from getting a cheap phone with the carrier bundle deal, then dump the service and getting a new carrier before they've paid off the iPhone. Unlocking is not done by Apple, so they cannot help you. They have, however, helpfully provided instructions to get your iPhone unlocked, if possible.

How to unlock your iPhone for use with a different carrier - Apple Support

No ….. if you purchased an unlocked iPhone, it would be forever unlocked. Untile you provide relevant details, instead of blaming Apple, we can not provide advice.

Did you purchase the device directly from Apple?

Did you purchase the unlocked version?

montaser83 wrote:

i purchased it from a phone shop and I was informed yes it’s unlocked everywhere

Then it would appear the “phone shop” lied.

You will need to contact the original cellular provider to determine if unlocking is possible.

Or, return the phone for a refund, then purchase a truly unlocked device from Apple.

Paying in full is irrelevant.

If you purchased an iPhone that is locked to a cellular provider, only that provider can unlock it.

Exactly where did you purchase the phone?

Where are you using it?