Why should anyone believe in macro-evolution?

If you are of the opinion that qualifiers like "probably...most likely...etc" are not science you clearly don't know what science is. This explains why you are clearly unaware of the numerous experiments and observations which do clearly demonstrate speciation. These experiments and observations demonstrate speciation as a fact, as does the fossil record and the growing catalog of gene and protein homologies between species. Any good theory will generate hypotheses, though. This is where the qualifiers come in. There is also often uncertainty surrounding certain details of implementation of a process with absolutely no question as to the implications of the end result. For instance it may be obvious that two populations that used to be more or less genetically identical have significantly diverge from one another. That speciation has occurred would be obvious and unquestionable so there would be no doubt but what specifically mutated and how would be a matter for further research and so, initially, speculation. The speculation, which would be accompanied by the qualifies you complained about would be present to guide research by suggesting solutions to new questions posed by the new discovery. They may be wrong in part or in total, but usually something useful is learned by exploring the possibilities.