Why don't crowds in Japan bring signs?

They do and they don't.

Japan does not have the culture whereby you bring giant signs to wrestling show declaring your undying love for a wrestler or how much you hate another or mocking his or her catchphrase (I think we have all seen the infamous Kurt Angle "Three I's" where the speller said one "I" was for "idiocy" but couldn't spell it right).

Banner examples

The banners you will see on the balconies or the walls are sometimes bought to the event by fans and put up or the promotion will have bought them with them. Some venues have banned waving banners in the crowd due to complaints that the people behind cannot see. Standing up in your seat is also a polite request that some venues make that you do not do.

Like anywhere else the crowds do cheer their favorite wrestlers, it is a myth that Japanese crowds sit quietly. In reality they make just as much noise as anywhere else; they boo, they cheer and on occasion they have even rioted.