Contraceptives are not guaranteed to prevent pregnancy or diseases because it would be suicidal for a company to put a guarantee on them. Most contraceptives are not 100% effective against either pregnancy or disease even if they are used correctly. When not used perfectly, they can be ineffective. A company that made a guarantee would find itself in court very often, trying to prove that people used the contraceptives improperly and therefore became pregnant or got a disease. This would be very expensive and any lost lawsuits would be disastrous.
Contraceptives simply cannot always prevent pregnancy or disease. A condom can tear. It can fall off during use. It is relatively easy for a woman to forget to take an oral contraceptive. Oral contraceptives, of course, do nothing to prevent diseases. Other kinds of contraceptives have their own weaknesses as well.
Because contraceptives are not perfect, and because it would be very expensive to defend against law suits, contraceptive makers will never guarantee them to be 100% effective.