Coz they are both owned by the same company called Vitec plc.
No brainer that Gitzo will always be the higher end big brother of Manfrotto to cater to 2 different market, although they will be like SG Govt telling Indonesia that "no no, they are both different telecommunication companies and they are run commercially by their own".
1) Can afford, buy Gitzo
2) Cannot afford but wants Gitzo lookalike, buy some made-in-china value stuff (Sirui, Benro?)
3) Cannot afford and do not want made-in-china stuff, get Manfrotto or Silk or other brands suitable.