Why don’t we say both "Mashiv HaRuach U’Morid HaGeshem" and "Morid HaTal", always, all year round? They both are praise to HKB"H, not a request or expression of gratitude. We praise G-d for His abilities, it’s like saying there hasn’t been resurrection of the dead yet, so we can drop that Bracha for now. The separation between summer and winter is understood only when it’s a request, in "Barech Aleinu"- "V’Ten Bracha"/"V’Ten Tal U’Matar"?
The praise "Mashiv HaRuach U'Morid HaGeshem" is said only when it's currently apparent in nature. In the winter we see it, and so we praise G-d for it. Regarding "Morid HaTal", there are different opinions regarding its mention, some say that since the dew never stops it is unnecessary to recite it at all, and Sephardic Jews do recite it in the summer. But praise for the rain should not be mentioned if it is not currently relevant. This question was answered by Rabbi Dov Lior