because blitz has you making faster moves, and its usually more difficult to think at faster speeds, at least for beginners. (someone please correct me if this is wrong) im the same way. (although ive only played 3 blitz games- im just too slow)
Generally after playing a lot of rapid games as a beginner it becomes harder and harder to switch to blitz, which is why in OTB when you're starting out you don't begin with the time control that the GMs and other titled players have. Start with 15|10 games to learn how to move the pieces around, make sure that's intuitive, then play a bit of blitz, get crushed, that's normal, then use rapid to stop hanging pieces, work on getting faster in blitz, etc., maybe try a little bullet to prepare for time-scrambles, and that's how I train. Currently at around 1650 blitz and 1600 rapid btw.
because blitz has you making faster moves, and its usually more difficult to think at faster speeds, at least for beginners. (someone please correct me if this is wrong) im the same way. (although ive only played 3 blitz games- im just too slow)