Why is it difficult to design an effective speech recognition device?

Why is it difficult to design an effective speech recognition device?

First of all- because everyone talks differently. Vocalizations vary in terms of accent, pronunciation, articulation, roughness, nasality, pitch, volume, and speed. Speech is distorted by a background noise and echoes, electrical characteristics. Error rates increase as the vocabulary size grows. Vocabulary is hard to recognize if it contains confusable words. Language changes everyday, so such a machine would have to update constantly to keep up with those changes. Existing speech recognition devices have huge problem with recognising continuous speech. It is still too difficult for them to process such a huge amount of information (taking into account individual speech, speed, grammar, vocabulary) at the time as short as human brain does. It is also hard for a machine to tune off the environmental noises, whereas brain does it effortlessly and automatically. It is the matter of attention.