CCNP ENARSI- BGP Doubts

  1. Why we need iBGP ?
  2. Why can’t we use IGPs rather than iBGP?
  3. Why AD value of iBGP more than eBGP and other IGPs ?
  4. Why OSPF protocol is not used in Internet rather than BGP ? I believe link-state protocols invented earlier to Path Vector protocols like EGP and BGP ? But still BGP is chosen instead of OSPF/IS-IS ? why ?