I understand why authors feel frustrated when agents (or other publishing professionals) keep them waiting, especially when it could be indefinitely. But sometimes, especially for in-demand agents, they have to think about what matters for their work. Serving their existing clients is their work. Answering authors they’re not interested in is polite, but not ultimately important to their livelihood.
As an author, it’s easy to fall into a trap of feeling slighted that you don’t get back the care and consideration you put into researching representation. You may even feel like you deserve to lash out or scold publishing professionals who don’t even do you the courtesy of a response. But at the end of the day? At least in agenting, these folks will be taking a chance on you if they say yes, and they have to be reasonably sure about the selling potential of your work since they’ll literally be agreeing to work for free if they sign you.
If you start your relationship with them making it clear that you’re Difficult to Work With and Very Entitled, you are not showing them who’s boss or making them think you’re a big shot who deserves their attention. You’re showing them that you don’t have the patience or humility you’ll need to survive in the publishing industry. Plus, just for your own good, you should never burn a bridge with someone in publishing just because they rejected something of yours. People take other positions in publishing all the time, so you never know when you might find yourself facing someone you insulted in a publishing decision, or maybe you want to offer them another project they would have otherwise taken if not for your attitude.
Anyway, short version: yes, it sucks to get ignored, but no, you are not entitled to a response (even though yes, it’s nice to get).