Even gpt-5 on "high" reasoning effort (which is likely higher than what people get in the Plus subscription; that's most likely "medium") is very, very low compute compared to the top runs behind IOI/IMO solutions.
Thermodynamics is the law of laws, unless they invent some kind of ultra-efficient, almost magical computers to run these systems, it's simply not economical yet.
The reasons could be that it's new (they did say they plan to release eventually but not soon), or that it's too heavily scaffolded for the task and not sufficiently general.