Why discrete time signals are defined as sequence of numbers?
Samples of (regularly sampled) discrete time signals, like pixels of a digital images, are sequences: they are unitless on the ordinal axis (albeit time or space), and unitless in amplitude. A sequence is

an enumerated collection of objects in which repetitions
are allowed and order matters

This is close to what people call raw data.
However, to make a genuine &quot;signal&quot; or &quot;image&quot; object (meaningful in the real world or for devices) that can be better interpreted or manipulated, it is important to keep ancillary information, like: sampling rate, quantization, time-space coordinates, size, channels, amplitude units, calibration parameters, etc. Those additional parameters are found for instance  in the header of most digital sound or image raster formats, and then followed by the unitless digital sequence.
Below is an example from the WAVE PCM soundfile format (converted in png for display). The Discrete signal is typically in the bottom &quot;data&quot; sub-chunk, yet requires the above information to be read, listened to, analyzed.