The tension in a massless string is considered to be the same across the entire length of the string due to the idealized assumption that the string has no mass. This assumption allows us to neglect any gravitational or inertial effects on the string itself.
When a massless string is subject to a tension force at one end, that tension force is transmitted instantaneously throughout the entire string. This is because there is no mass to cause any delay or resistance in the propagation of the tension force. Consequently, the tension force is uniformly distributed along the entire length of the string.
In reality, physical strings have some mass, and as a result, there can be slight variations in tension along the string due to its own weight and other factors. However, these variations are typically negligible and can be ignored in many situations where the mass of the string is much smaller compared to the loads applied to it.
The assumption of a massless string with uniform tension is often used as a simplification in physics problems to focus on the effects of other forces or objects connected to the string.