I haven't seen anyone mention this previously and it is a point I've often pondered. My own thought is that many churches fear presenting baptism as in any way important because mentioning it may lead to thoughts of baptismal regeneration. It is all-too-often thought of as an optional extra. In some paedobaptist churches it is not mentioned because it is assumed that people were "baptised" as babies. But in immersionist churches it is also not often mentioned, but comes up as a next step and as a prerequisite for local church membership.
I was raised in the Baptist church and when the gospel was presented, a was often quoted:
"Sirs, what must I do to be saved?"
They replied, "Believe in the Lord Jesus, and you will be saved
It is interesting that those who taught us that belief alone was all that was required did not seem to know that a few verses later tell the story of the Philippian gaoler and his household being baptised, showing that baptism was part of Paul's message.
I think many Christians today are afraid of baptism because they think it may cause division, so they think the wisest course of action is to omit it altogether.
Btu why is it left out of gospel presentations? I think that perhaps the main reason is that these summaries are intended to be used in many different Christian denominations and in interdenominational contexts.