Because of Quora’s habit of applying answers to unrelated questions I need to clarify that the question I am responding to here is “Why is Newton’s second law written in the form F = ma, since a more accurate form based on the formula F = dp/dt should be F = m dv/dt + v dm/dt = ma + v dm/dt, since according to the theory of relativity, mass also changes, as does velocity?”
Well first, Newton’s second law is written in the form F = ma because that is essentially the way he wrote it and if it wasn’t written that way it wouldn’t be Newton’s law. The form F=dp/dt (which is more general but not more “accurate”) was known to him but he didn’t need to use it because he was referring to the special basic case of an object of a fixed mass (as opposed to something like a rocket expelling massive exhaust) and he chose to take just the simpler special case as his starting point and deduce the more general as a consequence.
He did not write his laws in Lorentz covariant form because he was unaware of almost everything about electromagnetism and radiation, and so had no reason to expect that the correct laws of physics were not actually perfectly Galilean covariant. (But in any case, as described in more detail in other answers, both the Lorentz covariant laws of special relativity and those of General Relativity can be expressed in a form which includes an equation of the form F=ma for appropriate definitions of F,m, and a).
But your claim that “according to the theory of relativity, mass also changes, as does velocity” is based on a concept of “relativistic mass” which is not a well defined property of an object and has long been abandoned as misleading and not useful.