Question

why alkenes and alkynes are more reactive than alkanes

Answer 1

Because these have weak bonds than alkanes which can be broken easily

Answer 2

I'll try to keep it as simple as I can.

You see, Alkanes are the worst nightmare of an organic chemist because they are dead ends. You cannot go anywhere from them (except for methane. It usually takes one to the final destination). Alkanes have nothing with them to react with. They don't have polar bonds. They don't have poles because of difference in electronegativity. Basically, Alkanes are boring.

Now the Alkynes on the other hand are interesting af. They have loads of electron cloud density on them and they are inviting the electrophiles(those who carve for negative charge), to come and attack 'em