First of all, it looks fantastic! Of course there are other, less visible reasons as well.
The most obvious difficulty with buying an inexpensive ready-made frame is that the art will not likely fit, and will never look right even if you find a way to squeeze it in. But there are bigger problems.
Most ready-made frames will provide no means for holding the artwork in place inside the frame. Tapes of all kinds are unsafe and will probably fail at some point. A good custom framer will use appropriate mounting techniques that should hold the work in place forever, will not cause buckling, and will definitely do no harm to the paper or other support material.
There is almost certainly inadequate air space under the glass to prevent mould formation over time. A custom-made picture frame should include at least 8 ply of air space, the equivalent of a double mat, for proper air circulation.
Are all the materials acid free? It is impossible to tell now, but in several years the edges of the mats may become brownish in colour, indicating that there has already been significant damage to the artwork. The best choice is pure cotton rag, which is naturally and permanently acid and lignin free. Wood fibre mats, although they may be described as acid free, will eventually exhaust the calcium carbonate buffering they contain, becoming acid after an unknown amount of time.
Is the glass antireflective? It can be very disappointing trying to view a wonderful drawing or painting through a harsh and inescapable reflection. What about the level of UV protection? All glass will provide about 49% protection, but some will provide as much as 99%.
What about the frame itself? Is it actually strong enough to support the weight of the glass, resisting bending and twisting? Are the hangers secure enough? And what material is it made of? Many mass-produced frames from China are made of plastics and composites that contain toxic chemicals and heavy metals, while the best wooden frames are made from trees grown in certified reforestation projects.
Finally, unlike that low-priced option, an exquisite hand-made custom frame will provide the aesthetic delight of a well-designed package that fits and enhances, while protecting and conserving your valuable artwork for generations to come.