Question

why alkenes undergo electrophilic addition and not electrophilic substitution reactions​

Answer 1

Because the double bond in an alkene is a relatively electron-rich location. This can be visualized on the following image of E-2-pentene. In the image, the charge distribution is projected onto the molecular surface. (Red is positive, blue is negative)

Because the region of the double bond is partially negative, it makes sense that it reacts with a (positively charged) electrophile, H+. This gives a carbocation, which in turn will react with a nucleophile, e.g. Cl−. So this is the mechanism of electrophilic addition.