Because
1.the 300 Mbit is only a theoretical speed. You will get 80 to 120 Mbit max from it.
2. wireless in general is -by nature- known for being vulnerable for more lag and fluctuating connection quality whereas wired ethernet is rocksolid (when right cable quality is used; CAT 5e and +) since wires are not affected by interference from neighbouring AP's, DECT phones, static, concrete walls/steel in walls, etc.
3. 720P movies can have different bitrates. The higher the bitrate, the more bandwidth is needed (MB/s; do not mix up Mbps with MB/s, Google for online calculators that convert Mbps to MB/s)).