Can anybody tell me what the rush is all the time to retract the gear ?
The only time to rush might be in a rejected takeoff. Otherwise, there is no need to "rush" any time when flying a transport, especially when performing a go-around, but, like engine fail/fire drills, it should be done smartly, methodically & deliberately...as soon as the flaps are set and the "positive rate" (or some such similar SOP" call is made by the PNF, retract the gear. It's important to stay in sequence - set power, raise flaps then gear while achieving correct pitch. When completely settled, communicate with ATC, then the passengers.
The reasons to get the gear up are numerous - obviously the aircraft performs better without the drag! But go-around certification climb performance and where necessary, obstacle clearance, particularly with engine-out performance, are based upon the manufacturer's numbers which would include flaps set correctly, gear retracted and pitch attitude achieved, (normally at least >= 12.5deg).
Go-arounds are higher-risk maneuvers than many think. It's almost always unexpected and so is a surprise, (although I've never believed it should "startle" a seasoned well-trained crew). There are studies on this aspect of transport operation which examines both crew and aircraft performance; here's
one. Most FDM/FOQA Programs have events which help validate & improve the maneuver.
Hope this helps.