Because in her homeland it's a sign of respect. (No, seriously!)
We learn a lot about Shakra's cultural background throughout Silksong. She comes from a tribe of proud warriors who adhere to traditions of travel and combat. Where she's from, one's weapon is attached to one's name as a signifier of identity, and there are special rules around how one greets strangers on their travels. When meeting a fellow warrior, you say "poshanka" while presenting your battle-stance and weapon:
That stance is an important detail, because the physical way you greet someone says a LOT about your cultural principles. To give two examples: The bow is an ancient sign of deference, in which you make yourself smaller, look to the ground, and lean forward in a way that leaves you vulnerable to attack. All clear indicators of submission and trust. The handshake meanwhile originated as a greeting between warriors, in which you present your dominant hand to indicate you have no intention of drawing your weapon. An indicator of peaceful intention but not submission, as to maintain each warrior's pride.
Shakra's greeting takes the prideful aspects of the handshake to its logical extreme. In her tribe you show respect not by lowering your guard, but by raising it. This implies acknowledging the skill of your fellow warrior is considered more important than establishing yourself as peaceful or friendly. The greeting declares: "I consider you dangerous enough to feel threatened. Here is my fighting-style and weapon of choice, the two things most integral to my identity."
With this context in mind, we can connect the dots as to why Shakra lowers herself to speak to Hornet. In nature, eye-level is an indicator of size that translates to threat and authority. Having to look up at someone while speaking to them makes you feel small, establishing a difference in power. For a tribe of proud warriors who base respect around acknowledging one another as dangerous, looming over a fellow warrior so that they feel small could be perceived as a challenge to that warrior's pride.
Thus in Shakra's tribe it became common practice to lower yourself to another's eye-level while speaking, as a physical indication that you consider them an equal and intend to speak to them as such.
