I don't know. Him screaming "ASSEMBLE!" not only would have felt incredibly cheesy, but it wouldn't have messed with where Cap was at mentally and physically in that moment. I think Evans made the right call.
Cap had just gotten beaten down by Thanos after wielding Mjolnir and giving him his best, Thanos' army was bearing down on him, and he was still gonna put up a fight, despite barely being able to tighten his broken shield around his wrist. He knew he wouldn't win, but he was sure as hell gonna take out as many of those fuckers as he could before he goes. Then he hears Sam's voice over the radio, and the portals open, and his allies come to his aid. His "Assemble" was perfectly delivered. The anger and determination in his voice sold it.
yelling is dramatic and all flash and no substance, but nothing puckers up your butthole more than when your parents come at you with a calm, measured voice like "you know you done fucked up, right?" Cap was doing that to Thanos. Lol yeah, it's often drowned out by the crowd reaction, but I still prefer it to a bombastic yell. That gritty determination in his voice really worked for me, personally.
Edit: it actually reminds me of Christopher Reeves', "General, why don't you step outside," from Superman 2. Not bombastic and over the top, but you felt that Supes was about to tear Zod a new one.