Life / Polish interment practices...cemetary burial. [17]

I recently visited Poland, north and east of Warsaw, trying to find evidences of relatives who had not emigrated to the USA at the turn of the twentieth century. In addition to searching civil records, etc., I also visited several cemetaries on a search for names and dates (and photos, in some cases). There were very few old gravesites in cemetaries in the "countryside", that is, earlier than about 1918. I realize that two World Wars swept over this ground, but I also got the impression that it was common practice to put graves right over pre-existing sites, especially if older graves were not "kept up". The folks I asked about this in Poland did not seem to have a ready answer, even in one mortuary establishment! Was this common practice, and is that why cemetary scoping for old relatives was so barren of results?