why does my iphone 14 ringer volume go lower the more it rings

It is designed to lower the ring volume as soon as it recognizes that you have heard it. You can turn that option off if you want.

If Settings > Face ID & Passcode > Attention Aware Features is on, the ring volume will automatically decrease as soon as the phone recognizes that it has your attention. Also see: Turn Attention Aware features on or off on your iPhone or iPad Pro - Apple Support