Because it assumes that framing is performed by the underlying protocol and that only one RTP packet is to be carried in one PDU of the underlying protocol.
I hope you know that UDP is message based protocol. It carries only messages. Whatever the data is comes through UDP it will cut the data in to his message size and will carry to the destination.
This is the typical application with UDP (or AAL5) as the underlying protocol. Since most applications currently envisioned do not need framing, it would be a waste of processing and bandwidth to add one.
This is covered in detail in the section RTP over Network and Transport Protocols of the spec.
If RTP is used with a protocol that is not message-based (e.g. TCP) or if it is desirable to carry several RTP packets in one lower-layer PDU (e.g. for aggregation of streams) it is trivial to define a profile that prefixes the RTP header by a 16 or 32-bit length field, depending on the desired tradeoff between overhead and maintaining word alignment.