1. Religious mysticism. The murkier people are on concepts like thinking, consciousness, and intelligence, the easier it is to claim they include some metaphysical aspect. Since you cannot actually pin down the metaphysical aspect, they must claim it is because you cannot pin down the physical aspect.

2. People do not like feeling less intelligent than other people, so they try to make the comparator ill-defined.

#2 is not relevant, and it also seems basically untrue.

So your belief is that the global scientific community broadly agrees that "intelligence" has not been rigorously defined because the global scientific community is trapped in religious mysticism?

I am going to be honest, and I'm not saying this as a jab - this is starting to sound completely disconnected from reality. The people who study intelligence are not, as a rule, mired in metsphysical hand-waving.

Huh? You asked, "why is there broad consensus today that intelligence is ill-defined?" That's what I answered. Did you mean to ask a different question, "why is there broad consensus among people who research intelligence that it is ill-defined?" Which kinds of people are you talking about? The information theorists? The machine learning researchers? The linguists? The psychologists?

The information theorists generally agree it has a precise definition, though they may choose different ones. The machine learning researchers typically only know how to run empirical experiments, but a small group of them do theory, and they generally agree intelligence is low Kolmogorov complexity. The linguists generally agree it cannot be defined, in the nihilistic sense, but if you posit a bunch of brains, then words have meaning by being signals between brains and intelligence is moving the words closer to the information bottleneck. I don't know what the psychologists say on the matter, though I wonder if they have the mathematical tools to even say things precisely.