[kolpa]

Those diphthongs have the same sound as combining the sounds of the two letters used to spell them, though.

[DonaldPShimoda]

The sounds may be similar, but they are not identical. Diphthongs are considered to be single segments phonologically, and some pronunciation changes can occur in "blending" the component parts together.

But I'm not sure I would go so far as to say all of the diphthongs should be counted as separate "vowels" in the language.