I mean, 'junk' DNA is just the noncoding regions. It contains everything that would look like a conditional jump to us CS folks, or a gate to EE folks. The coding regions are closer to .rodata; literally tables of the protein chains.
The relative lack of non coding DNA in other, simpler cells doesn't tell us that we don't need ours. It's equally possible that our DNA is just making significantly more dynamic decisions.
But the fact that some have more non-coding DNA doesn't really fit that idea. Unless you think amoebas are secret masters of the world. The simplest explanation is still that it is neutral baggage of neither benefit nor hindrance and organisms can gain or lose it by chance.
Or their strategy is to generate proteins with less of a concern for how their environment is dynamically changing. Which more aligns with single celled and very simple animals.
And it's not necessarily that they have more coding regions. It's that they have a greater percentage vs noncoding. It's a proportional thing.
The relative lack of non coding DNA in other, simpler cells doesn't tell us that we don't need ours. It's equally possible that our DNA is just making significantly more dynamic decisions.