Hot pixels are more noticeable in long exposures because each pixel is allowed to collect charge for a long time before being read out. Any pixel with higher dark current or leakage can build up enough charge to appear bright.

In live view, the sensor is read and reset repeatedly for each video frame, often many times per second, so charge does not accumulate for nearly as long. That prevents many hot pixels from becoming obvious.

Also, live view often does not display every sensor pixel one-for-one. The camera may skip, bin, or downsample pixels to create the preview, which can hide isolated hot pixels that would be visible in a full-resolution long exposure.