Do riffs/chord progessions need to be based off a certain scale ?

I listen to a lot of metal , and notice that a lot of riffs seem to be , if anything , based off the chromatic scale . I was wondering if , especially for thrash metal , riffs have to be based off of chords . Take , for example , part of the beginning to master of puppets :

E |--------------------|--------------------|---------------------|
B |7---6---5--------|---------------------|---------------------|
G |7---6---5--------|---------------------|----------------.----|
D |7---6---5--------|---------------------|----------------.----|
A |5---4---3--------|---------------------|---------------------|
E |--------------------|0-0-7-0-0-6-0-0|5-0-4-0-3-0-2-0--

The 2 last parts seem to be based off of either nothing or the chromatic scale . Do riffs/chord progessions need to be based off of scales , like solos ?

In short, no.

Riffs and licks can be anything. Whatever sounds good to you, will work.

It's easier to write a song and keep it in key most of the time and stray for some riffs that break the key or use augmented or diminshed notes/ intervals. There's no set of rules. It's more practical to stay in key and write all the riffs in key, but you don't have to necesarily do it all the time throughout the entire song.

What's strange is in almost every case, if it sounds good, its probably some kind of scale. But every note on the fretboard is part of a scale, so really if a solo you play has a note in it that doesn't go with the scale you were playing at first, odds are you've switched scales, which is fine. The way I see it, the guitar is just one giant scale.