Why do evolutionists say.....?

Not accurate, actually. In the absence of environmental selection pressures more variants of any given gene are viable, if you think of the peppered moth experiments, for example, in the absence of predation creating differential reproduction between members carrying the light colored gene variant verses the dark colored gene variant there would be nothing to remove one or the other from the gene pool or favoring one variant over the other so variation would be random. In large populations like ours the law of large number keeps the relative distributions of genes in a population around its average. Especially if there are no selection pressures at work on those variants creating differential rates of reproduction between carriers of each variant.