As someone who has worked on free Internet services, it used to be easier to make money from ads. A video ad view was worth nearly a dollar US. Audio ads were maybe 7 or 8 cents a listen. Now, a video ad view makes a few cents and audio ads are worthless. They likely did the math about how many audio ads they’d have to play on the phone in your pocket to break even and decided you’d hate it more than they would. Since content owners get just over half of what YouTube makes, they’d probably be pissed about seeing the drop in income too.
Feel free to hate YT. This was an economic decision at around the time when ad revenue had just fallen off of a cliff.