Most of the courts don’t think they are. Early rap beats used lots of samples. Some of the most popular hip hop songs made $0 for the artists as they had to pay royalties on those samples.
No one cares about what the law thinks about art though, particularly for personal consumption or sharing with a small group. Copyright law doesn't even pretend to be slightly just or aligned with reality.
And those sampled instruments were copyrighted by the manufacturers: YamahaGS, RolandXM, Alesis, Korg, etc. Early hip-hop were sampling disco and R&B records and got roundly slapped once they become popular for money to be involved.
That doesn't answer the question if music composed of samples is considered original, though. It is merely a legal ruling about some music from a certain time period that a lot of people would consider original.
