That’s not in fact true. Something has/is having an impact upon Biden’s numbers…both nationally and in a lot of the early states. He was running at about 40% in mid May nationally, now he is at 30%.
It’s a little harder to see on state level polling, because there isn’t as much consistent polling everywhere. But the trending is quite noticeable in NH, for example…and to a lesser degree in IA.
Sanders took a bit hit when Biden declared, and has held firm or slowly dipped ever since, largely at the benefit of Warren (who is the only candidate that is showing a positive longer term trend). Harris got a big bounce from the first debate, but is on a crash and burn trajectory ever since. Buttigieg got a bounce in May, and has held pretty firmly there, while O’Rourke has been steadily sliding down pretty much since he announced.
RCP puts it in a graph for you to easily see, at least nationally.