Because he understimated Goku's attack potency and didn't thought it were going to be that much dangerous as it actually were. But the one who is inciting Goku to eat a senzu bean after the incident is Cell and remarks how much strength he has lost, even if he got weaker it was so minuscle that...
Let's assume that is Perfect Cell the one that killed Trunks, how things would have gone for him?
Cell had to prepare himself and is visibly shown struggling to do that in order to receive those attacks:
Gohan despite of knowing that Cell has gotten stronger was amazed that it left zero...
In Super it is by Shin saying they used so much power on the Final Kamehameha. But is a thing that not always apply and it's basically headcanon.
What about the others (Freeza and Cell)? When it comes to the barrier?
Android 17 is clearly the exception as his power never drop. But what about Freeza, Cell or Vegetto?
100% Freeza had a poor stamina, yet he is able to make one and his power doesn't decrease until the end of the fight.
Cell's power is never stated to have decrease post regeneration and nobody...
Neither in Z or Super, they're rivals. Goku also hide the SSJ3 in front of Vegeta because how insignificant he considered him and he just cares about getting stronger or never losing, but doesn't have active Vegeta on his mind.
What do you think? I would have liked for him to return in the Buu Saga and explain why he wasn't dead as opposed to just re-appear to get defeated and everyone he killed would just be resurrected.
Zenkais can't be inflicted by self damage as stated by Vegeta:
Do you think Toriyama retconed this or forgot about this line? Therefore this line would not have any more value from that point on?
It's also a weird point to be brought. Cell was still two steps ahead of Goku and visibly speedblitzed him, so him not saying that doesn't deny those two things.
Majin Buu would be quite dangerous in the tournament, we manage to see Dyspo having trouble with gum fighters that can use their body like Buu can, so at least, I don't see how he couldn't at least take him out. On top of him being able to totally restore Goku's shape once he was defeated by...
This is a better and more logical approach, hadn't think about it.
Cell doesn't have any problem fighting Goku in regular conditions & Trunks is willing for all of them to attack when Goku is recovered. It's clear that the narrative treat heavily dropped Cell (in power) relevant to FP Goku.
Still doesn't hold. He is just knowing that Goku's chances can slightly increase if he do that, not randomly estimating or knowing about a portion of a power he's not emitting. Specially when he plan to attack Cell at the moment Goku is given a senzu.
You're quite uncapable today.
Trunks can't read hidden power. So he shouldn't know anything about it and just rely on the power Cell is displaying at the time.
If he knows about it, why not just bet on Gohan to do the job and defeat the weakened Cell?