your elders because they worked hard to build and maintain the society
in which you live, the benefits of which you now enjoy.
You respect your elders because you are called upon to treat everyone you meet with respect unless they show themselves undeserving of it. The next level down is mere politeness. Blatant disrespect should be reserved for when it is absolutely necessary, or it loses all moral force.
You respect your elders because Some times your elders may know what they're talking about.
You respect your elders because someday you will be one of the elders in question, and if you spent your youth creating a society where disrespect of elders is the norm, you will be extremely sorry.
You respect your elders because every human civilization up till now has considered it a good idea. This likely means that they tried the alternative and found it wanting.