Video encoding has several tradeoffs:

• Bitrate
• Resolution/frame rate
• Perceived quality
• Computational complexity of encoding
• Computational complexity of decoding

The cell phone encoding chips for video encoding on device make sacrifices to preserve speed of encoding and preserve battery life (higher computational complexity costs more processing cycles and tends to use more power). So it's simpler encoding, in exchange for inefficient bitrate compression.

YouTube (and all the social media sites) have huge server farms with highly specialized encoding chips for making the videos more efficient with bitrate for quality. That makes sense because videos designed to be watched millions of times could benefit from even a very slight improvement of bitrate in exchange for a one-time cost of complex encoding. It's also why YouTube tends not to convert to AV1 (very efficient in bitrate for quality, but computationally complex to encode) until a video has a few hundred views, because it's not clear whether that tradeoff is worth it until they know a lot of people will be watching it.

Netflix customizes even further for a per-video basis and looks for even more specialized tricks on a scene-by-scene basis, because every single one of its videos only needs to be encoded once for each quality/format but will be watched millions of times.

In other words, it's like any other engineering problem. The engineers choose different tradeoffs based on context, which means that the cell phone applies a different set of tradeoffs compared to the social media site's server farm.