Why do you keep the silent letters in writing, but omit in speaking English?

That they weren't silent at some point in the past, and spelling didn't change as quickly as pronunciation.

spelling is based on old language pronunciation. When the spelling of, e.g., veulent (Latin volunt) was fossilized around, we can see that the vowels were no longer pronounced as o and e were during that time period and were thus changed. 

The consonants remained the same. Now, despite the fact the nt isn't pronounced, we know it was in the Latin and can confidently guess it was in Old language.That they weren't silent at some point in the past, and spelling didn't change as quickly as pronunciation.

spelling is based on old language pronunciation. When the spelling of, e.g., veulent (Latin volunt) was fossilized around, we can see that the vowels were no longer pronounced as o and e were during that time period and were thus changed. 

The consonants remained the same. Now, despite the fact the nt isn't pronounced, we know it was in the Latin and can confidently guess it was in Old language.