Because we as readers of the story always take for granted that what happened in the story was either necessary to happen, or supposed to happen for some reason. Very few people actually ask the question of whether any of it was actually necessary.
For example, did Isaac even need to bless his Jacob or Esau? Isaac was never blessed by Abraham before Abraham died and everything that was supposed to happen according to God's plan...continued to happen according to God's plan. In fact Scripture even tells us that God is the one to bless Isaac, hinting that the only blessing that truly matters comes from God directly.
Genesis 25:11
יא וַיְהִי, אַחֲרֵי מוֹת אַבְרָהָם, וַיְבָרֶךְ אֱלֹהִים, אֶת-יִצְחָק בְּנוֹ; וַיֵּשֶׁב יִצְחָק, עִם-בְּאֵר לַחַי רֹאִי. {פ} 11 And it came to pass after the death of Abraham, that God blessed Isaac his son; and Isaac dwelt by Beer-lahai-roi. {P}
So it's entirely possible that this idea of blessing is something the Patriarchs decided to do culturally, but it might have little religious or divine value. Therefore, Isaac didn't need to bless anyone for God's will to continue to manifest, therefore God didn't see any real need to intervene with the blessings.