Why the short PCR product is not suitable for direct sequencing?

First of all, determining the purification and accurate quantity of short PCR products is very difficult and the general PCR products purification kit require products to be longer than 150bp, therefore we need the PCR products used for sequencing to be no less than 150bp in length. Secondly, it is owing to the limitation of sequencing itself. For example, sequencing a 100bp PCR product with the sequence of two primers about 40 to 50bp and some unreadable bases at the beginning of the sequencing being removed, the useful sequence that can be really obtained is no more than 30-40 bases. Therefore, the short PCR product can only be cloned and then sequenced.