Manufacturers offer different solutions to mechanical vibration/isolation, spurious reflections, scatter, noise, jitter, error recovery, analogue output stage, power supplies and lots of other factors.
These 'solutions' may include a 3rd party/OEM DVD transport costing a few dollars or an in-house engineered or modified design (like Rega and Naim).
Analogue output stages can incorporate opamp chips (of varying quality and price), discrete components or even valves!
Power supplies can vary depending on the quality of capacitors and transformers etc.
Component matching can be nil or painstakingly thorough. Engineering tolerances too.
Expensive light absorbent paint can be used to prevent spurious reflections/scatter or the manufacturer can ignore this and leave lots of bright shiny edges and surfaces in the effected area.
Similar arguments like "they all sound the same" or "all it has to do is turn at 33 1/3rd rpm" were levelled at turntables but despite universally accepted RIAA and (at one time) DIN 45500 standards, no-one could reasonably say all turtables sounded alike.
Two CD players from the same manufacturer can sound different and often the best ones were budget-mid priced models rather than top of the range versions.
Too many variables to simplify everything to one component being responsible for the 'voicing' of a whole player.