Yeah, OP literally said that they weren't blocked when using Vivaldi with uBlock Origin, you were the first one to mention the builtin adblock (which is detected by YouTube).
Again: to use YT, you have to disable the builtin adblock and use only uBO. That's in line with OPs statement.
Sorry if I misunderstood you, but this feels rather easy to answer: because you are being paid to write the code. Spotify doesn't pay anyone to write music (well maybe they technically do for some ads or something, but it's definitely not how they acquire more music to add to the library), they just pay for streaming rights on music that was somehow already independently produced. And tiny unknown musicians have no leverage to negotiate better terms than what Spotify offers.