I suspect they do that to eliminate any fallacious arguments over what you are suggesting is an "ancestry" test. DNA testing essentially helps to identify the genetic composition of one's DNA and where that DNA might have originated at some point in the past. "Ancestry" is far more complex.
I have trace amounts of Jewish, central and north African, and Indian genetic markers and a significant amount of German and British genetic links, but I identity as American. I speak American English, eat American food, listen to American music, celebrate American holidays, etc. I have moved on from my "ancestry" to a new reality.
My DNA does not entitle me to claim lands in Poland that were once part of Germany and seized from my family after WW2? Neither does the historical record that proves the lands were seized. My past ties to that land conquered by the Russians and given to the Poles were severed at the end of WW2. There is no inherent right to land lost as a result of a conflict.
What is surprising is that people want to use the argument of prior possession, and both can make that argument depending on how far back in time you wish to go.
When the British gave the Jews sovereignty over specific lands in Israel, that Britain gained control of after WW1, they did so, as every other conqueror has done including the Arabs during the Arab Conquest and the Ottomans.
The British saw the Arabs and Jews as having irreconcilable differences and so provide each with a "homeland". It was theirs to determine by right of conquest.
In creating a Jewish homeland, the government of Israel determined that it would accept as citizens, Jews from all over the world, who identified as Jews. Since Judaism does not have the same emphasis on conversion as Islam or Christianity, most Jews have some ancestral connection to Israel, though given how long it had been since they lived in Israel, the amount of Semitic DNA varies greatly by individual.
However, it should be noted, the practice of the Jews is not unique to Judaism. Islam employs similar practices and so it seems those practices are common in the Middle East. Dating back through history, because both Judaism and Muslim are tied to "theocratic" states... Ancient Israel and the various Muslim states, religion more than ethnicity defined citizenship. A non-Arab Muslim could become a citizen in an Islamic state and non-Semite Jew could become a citizen in Ancient Israel. While a non-Muslim Arab or non-Jewish Semite would not have the same rights in an Islamic Arab country or Israel.
In simple words, religion more than ancestry has long determined citizenship in the Middle East, not just Israel.
