States like Israel formally define their borders via mechanisms like peace deals that include bilateral recognition of agreed upon borders, like Israel has with Egypt and Jordan.
You can't realistically define borders properly otherwise, trying to define borders via unilateral declaration doesn't really work when there are 2 states next to each other, the borders ultimately need to be negotiated bilaterally between those states with both sides recognizing the same borders.
What other states do refuse to even say what they themselves consider their own definitive borders? I could find China/Taiwan and North/South Korea. In the latter case, they both claim it all, China keeps some thing vague like the "Nine-Dash Line", and Taiwan claims "all of China" because outlining their territory as just their island would come across as claiming independence or whatever.
In the case of Israel it is because they (well, the right-wing extremists that are in charge anyway) are belligerent, don't view themselves as bound by agreements with anyone anyway, always want to keep the option to grab more. No need for mental gymnastics.