So I decided to revisit Dragon Ball Z because why not. And I can't seem to find a good answer to this - why do the fighters let each other power up/transform? It seems to take a great deal of time - and, much of the time, they realize in the middle of it that the opponent's power really is climbing up, and that it isn't a bluff. And yet, they tend to stand by and let the opponent scream themselves to next level instead of attacking.
At first I thought the assumption is that it's all actually happening very fast, and is slowed down for the viewer's pleasure - even if the amount of reaction and exclamation that happens in the meantime seems like a stretch.
But, in episode 40, the following happens during Frieza's third scream-transformation:
Frieza starts screaming.
Vegeta tells Krillin to beat him up to near-death. Krillin expresses apprehension, then attacks, but not with full power. Vegeta insults Krillin and tells him to attack with full power. Krillin attacks. Vegeta gets damaged and makes a parabola for the ground, thinking about his future power as a Super Saiyan. Gohan looks on and asks "what happened?!" in surprise. Frieza keeps screaming. Piccolo yells at Gohan to get away. Gohan agrees, looks at the screaming Frieza, picks up Piccolo, and starts flying off with him. They have a short conversation about whether Piccolo is OK. Frieza keeps screaming, plus a little laughing and levitating small rocks. Piccolo and Gohan have a conversation in which Gohan mentions Dende, and how Dende can heal Piccolo. Krillin lands next to them. Vegeta and Dende, meanwhile, are having an argument because Dende isn't sure he wants to heal Vegeta. Vegeta reacts with anger because that's his only mode. Dende explains why Vegeta doesn't deserve healing. Vegeta objects. Dende flies off, Vegeta collapses. Krillin and Gohan are running (not flying) to get Dende, and chatting about Vegeta's power. Krillin explains what's up with Dende. Dende flies over them towards Piccolo. Frieza keeps screaming, plus now he's a little pink. Dende reaches Piccolo, Piccolo asks if Dende can heal, Dende says yes and heals Piccolo. Krillin and Gohan come up. Piccolo gets healed, and everyone is in a good mood while Frieza is screaming. Frieza, while screaming, is having an epiphany about how Dende can heal people. Piccolo remarks how this is amazing, and has a short conversation with Dende about whether this ability is only unique to some Namekians, or if everyone can do it. Krillin, appropriately, says "never mind Dende," and calmly explains to Dende how Vegeta may be a bad guy, but he needs to be healed. Dende is having a moral conflict about this. Piccolo tells Dende to do it for him. Dende relents.
Frieza explodes and completes his transformation.
Now, whether any of them attacking Frieza would have been any good is debatable. Piccolo got healed, but even he would have not been strong enough. But I think it at least shows that transformations aren't implied to be instantaneous.
So is there an in-universe reason for the transformations usually going uninterrupted? The stakes are often high. Not everyone who waits is some honorable fighter for whom etiquette is everything - plus that seems weird when the other guy wants to kill all your friends and maybe everyone on your planet.
So why do characters wait? Have any of the creators of the Dragon Ball universe commented on this?