??

Because when they are not speaking Catalan, they speak Castillian Spanish.

Wrong question.

In English, the sound "th" like in "think" is also a lisp, the same as in Spanish the sound z like in "zapato" (shoe).

But in Barcelona many people speak the local language, Catalan (which is similar to French) as their native language, and Catalan does not have this sound, this "lisp" as you call it.

People who have lisps have speech defects.

People from a whole city don't have a speech defect.

What makes you think the whole city of Barcelona has lisps?

becauth they thpeak in a thuper thpethial way.

??