Why does that matter? The point is you can combine elements and create compounds that are drastically different to the components with different properties.
Obvious one is water. H - explody. O2 - burny. H2O - puts out fires, quenches your thirst, no longer a gas, not very reactive.
Because corrosive is way different than toxic. Particularly, acid/base corrosion can and will be neutralized by our body once sufficiently diluted. Toxic compounds, especially heavy metals, stay toxic even if diluted.
The point is, that making corrosive properties go away is easy and very typical, whereas toxic properties often have a habit of not going away after it enters the body.
Most corrosive substances release their effects on the surface they contact you, but toxic effects, especially the problematic ones, are way deeper.
The distinction is fairly important, especially regarding heavy metals.
Like hydrofluoric acid, for example. It's fairly easy to not notice you spilled some on your hand/arm until after it soaked through your skin, upon which the issue is systemic toxicity or at least bone damage. In contrast, sulfuric, nitric, and hydrochloric acids at any concentration up to azeotropic just cause burns and, for sulfuric acid, dehydration. There are no acute systemic risks from all but large-area burns, and even then it's mostly due to fluid loss which can be compensated for. (Yes, there are later risks due to loss-of-skin and skin graft surgery, but none of those complications try to have you pronounced dead you within a few hours of the spill.)
