RAID0 can increase both sequential throughput as well as random-like I/O IOps. What it can't do, is reduce read latency. That ultimately is bottlenecked by the seek-rate of the HDD. The mechanical part that is the cause for virtually all 'slowness' on modern computers.
When benchmarking, you get figures like:
(
source)
This is a rather extreme scenario: reading 4KiB from random locations. Not the most extreme - 512 bytes random read across the whole drive capacity, is the ultimate test. As you can see the only HDD in the list, the fastest desktop-class also, the Velociraptor appears to have erection problems of some kind.
Seriously, this is the weakness and huge bottleneck of HDDs. HDDs can read and write fast, but they have to SEEK. You asked "What makes it sequential". Its not really being a question of sequential or not, but to what degree is it sequential. It could be 50% or 20%. Huh, you don't understand? You will
100% sequential:
Read sector 1, Read sector 2, Read sector 3, Read sector 4, etc...
100% random:
Read sector 1, Seek to sector 47839275, read sector 47839275, seek to sector 27395, read sector 27395, etc..
In reality, when writing one big file (copying) or other single stream of I/O, it will be highly sequential. It may have a few seeks, but otherwise its pretty much read and write for the HDD head which it can do very fast. But if it has to seek alot, which happens countless times on a desktop, this can put an extremely fast system down to the point of applications not responding and user input being ignored, until apparantly it finished what it's doing and it still reacted to whatever keys you pressed. The supermodern quadcore cpu can't do anything, because its waiting on the HDD to seek. Even 1 seek will mean the CPU has wasted 30.000 cycles or Hz it could have spent doing work, but couldn't because it had to wait for the HDD.
With many seeks, the CPU can only do 1-4% work and has to wait the other 96%+. SSDs can finally solve this bottleneck, by removing the last mechanical component of modern storage.