Being short is a disability, and as a tall person, it's your responsibility to make sure short people don't get even more disabled by having to look up to speak to you. Doing this can damage their neck. Standing up while talking to short people also makes them feel threatened and unsafe so kneeling down not only prevents them from damaging their necks but also makes them feel more safe. Not doing this means you don't respect short people, you're mocking their disability, you're looking down on them and that's ableist, so be the bigger man and kneel.
Thank you.