Why is the Tm defined as the temperature at which 50% of dsDNA has changed into ssDNA?
If you search for DNA melting temperature plots you will see they have a sigmoid shape. It's common for those kinds of plots to be described by the 50% value for example the therapeutic index and the 50% lethal dose of a substance (LD50). This is because at really low and really high temperatures all DNA molecules become dsDNA or ssDNA respectively so that information is not really useful. You could maybe talk about the first temperature at which DNA becomes 100% denatured but the curves are highly nonlinear at that region so it's difficult to quantify. The 50% is an easy way to compare different molecules since that region is usually linear and easy to compare in a meaningful way (basically all molecules have the same ends of the sigmoid shape, but it's the middle part that changes the most).