One of the main reasons Phineas and Ferb is so popular among fans is that of its humor and references to other popular media. However, there are some things about the show that seem a bit odd and questionable. For instance, why does ferb not talk?
Despite being the step brother to Phineas, Ferb rarely talks. He does occasionally chime in for comedic effect but usually prefers to be more hands-on with the building side of their inventions. He is also more of a laid-back type than Phineas and does not like to flaunt his smarts in front of others. During one episode of the show, he even tries to combine with Jeremy (voiced by Maulik Pancholy) in a similar way that P&F fused with Isabella but was unsuccessful because only the parts of them that touched were combined.
It is known that Linda Flynn-Fletcher (voiced by Caroline Rhea) knows about Phineas and Ferb's inventions but does not disapprove of them. This leaves many questions as to why she never tries to bust them or backs up her daughter when she tattles on them to her mother.
One theory is that she doesn't want to risk getting them in trouble with her husband or parents. Another is that she doesn't feel the need to do so since the inventions that Phineas and Ferb create are mostly harmless. The fact that they are helping to fight Doofenshmirtz evil in their spare time also helps to keep her quiet about their activities.