There are any number of reasons why people might want a rematch, even if Lopez clearly won. It was a close fight. Lomachenko was a top p4p fighter with a pretty vocal following (particularly online). Lomachenko had only lost once before in fairly controversial fashion. Lomachenko is still an elite LW. It's an in-house fight (now that Lopez appears to have dropped any delusions about going with Triller). The reasons can go on and on.
I do fail to see how someone could see Lomachenko winning easily on that particular night if he just started a little earlier. It was near consensus before the fight that Lopez was going to be extremely dangerous for the first 4 rounds and that Lomachenko was going to ease into the fight slowly. Now I agree he was overcautious, but the gameplan, at least on that night, was always going to involve Lomachenko easing into the fight and staying away early. When Lomachenko did turn it up, he wasn't exactly dominating from round 7 onwards even if he was getting rounds, and Lopez still had enough in the tank to emphatically win round 12 when Lomachenko really did go all out.
Maybe Lomachenko can win a rematch easily with a completely different approach, but with the way Lomachenko was always going to approach the fight on that night, and the way Lopez responded, winning easily was never in the cards for Lomachenko, even if he started in round 4 or 5 instead of 7. In fact, there's a certain amount of credence in the thought that things might have gone very poorly for Lomachenko had he started earlier.