My Hero Academia anime is currently in the final phase of the Final War Arc, with the most recent causality being Toga Himiko. The villain lost against Ochaco but this loss led to the fandom accepting Toga for her shortcomings. However, some fans wanted to see more from her and this had to do with why she never used Deku's blood for transformation.
Firstly, Toga couldn't logically transform into the protagonist because she utilized his blood during the Shie Hassaikai Arc. Secondly, copying One for All wouldn't turn out as expected because the quirk is dependent on the user's usage of itself. So, even if Toga copied Izuku's appearance, she might have gotten nothing as a quirk: just an empty husk.
Disclaimer: This article expresses the writer's opinion.
My Hero Academia: Exploring why Toga could never utilize Izuku's One for All in battle

During the last episode of My Hero Academia season 7, Ochaco Uraraka tried her best to convey her honest feelings to Himiko Toga. However, due to the villain's rebellious behavior, things weren't looking so good. However, eventually, Ochaco reached Toga and conveyed her feelings, which stopped Toga's rampage.
After the fight concluded, the hero was nearing her death due to blood loss and there weren't any signs of getting a blood transfusion. So, Himiko used her quirk to copy Ochaco's appearance and demonstrated a blood transfusion in the middle of battle.

As heroic as Toga's sacrifice might be, could she have survived if she had copied Izuku's appearance? Fans might think copying the protagonist's appearance might give the villain access to One for All. However, things would go differently. Firstly, Himiko would need Deku's blood to copy his appearance.
She had already used the minute amount (which she acquired during the Provisional Hero License Arc) at hand during My Hero Academia's Shie Hassaiki Arc when the villain lured the heroes away using the protagonist's appearance.

So, what if Himiko had enough of Deku's blood at the time of My Hero Academia's Final War Arc? In such a case, things would have turned out differently. One for All is a quirk passed on to another person only if the previous user has the 'will' to pass it on. All Might explained this before he passed on his quirk to Izuku Midoriya.
This is what made One for All a quirk of its own as it could never be stolen without a user's will. So, if Toga had an ample amount of the protagonist at hand during her fight with Ochaco, she could only copy Izuku's appearance as there was no will behind the passing of this quirk.
Analysis and Final Thoughts
Some questions were answered without any formal explanation, and this might be one of them. Due to One for All's dynamics, stealing it without the previous user's consent is impossible.
This was also why One for All was the only power that could go against, and kill, the demon lord, All for One. Similarly, this might also be why Toga could have never gotten her hands on this quirk.
Related Links
- Horikoshi already told My Hero Academia fans that Toga is alive
- My Hero Academia gave Toga a conclusion that Shigaraki never got
- Deku's Danger Sense in My Hero Academia proved how Toga truly expresses her love