In typical SEP aircraft, speed is much more easily controllable by pitch attitude than power. With draggy, often low powered light aircraft, speed response to throttle control may be laggy and ineffective, whereas changing the pitch attitude will instantaneously create an acceleration or deceleration as required. After that, the rate of climb or descent is related directly to the power setting. In a low / slow situation on an approach pulling up to correct the vertical profile and adding power for speed could result in a worsening situation by getting on the wrong side of the drag curve and even stalling on approach.
The pitch for speed technique is taught because it is far more appropriate for the handling properties of SEP aircraft than the alternative. These are the foundations of all your future flying, so it's a good idea to take onboard what the instructor teaches and not devise your own alternatives. After a short while you'll be moving the stick and throttle fairly instinctively anyway.
The thrust for speed technique is appropriate to certain other classes of aircraft that have rather different handling, performance and stability qualities compared to SEP. Don't overcomplicate things at your early stage.
And seriously, a top tip, be very wary of overconfidence and an I know best attitude. History has shown that it often leads to smoking craters.