Why mustn't the sample be dipped in the mobile phase in paper chromatography?
Dipping sample to a mobile phase would cancel all the purpose of chromatography as a separation technique.

Imagine properly performed chromatography as the athletic race, where all runners start at the same point and time. The finish order is by their average speed.

Now imagine a crazy version of a New York marathone, where runners start in  long sequence of runner batches. But nobody would  think about registration of runner starts, so there would be no idea which runner was the fastest to be decorated as the winner.

Back to chromatography, instead of chromatographic spots ( PC, TLC) or peaks (HPLC, GC), there would be useless, smeared, overlapping long bands.