That's wrong pronunciation. Many Koreans do same mistake, but 같아 is pronounced as [가타]. There's no reason for this. Because that's totally mistake. But in colloquial situation, you can pronounce like that in purpose, because that sounds comfortable and real natives. But not recommended to foreigners who study Korean as foreign language.

Originally the pronounciation of 같아 is [가타] as you know, but many people pronounce it wrong way. I think the reason of this phenomenon is that it is more convinient when people say [가태] than [가타], cuz when people say [가태], then they don't need to open their mouth big. However, 같아[가타] is grammatically correct :) Ex) 막아,잡아,달아 (correct) 막애,잡애,달애 (wrong)

Oh