That is only part of the reason. Most people believe we live in an universe with regular behaviors, such that observable phenomena, such as consciousness, depend on regular physical arrangements. This is a good reason to believe brains are all conscious. It is not a reason to believe only brains are conscious, but I don’t believe a wooden block which is painted red and white and has an iron ball glued to it has magnetism; this is just an analogy. It seems far more likely that consciousness depends on the particular material arrangement, meaning that substrate independence is wrong.

That’s not to say the computer is 100% not conscious, but its observable behavior is as likely to align with consciousness as the computers in the 90s. With this line of reasoning our credence towards it being conscious should be the same as the computers in the 90s, which is not very high for most people.

Intelligence on the other hand is obviously substrate independent. If one thinks harder one realizes our consciousness must at least at an early evolutionary stage have played a role in our intelligence, otherwise it would have evolved out. To me this just means we exist in a reality where the material arrangements supporting consciousness are biased toward intelligence.