The major difference is that treaties aren't subject to the Presentment Clause. Ordinary legislative acts originate in one of the houses of Congress and must be approved by both houses before being presented to the President. Some congressman or senator is on the hook for having written and sponsored that bill. International treaties are given to the Senate on a take it or leave it basis, and do not require any approval from the most democratic chamber of Congress: the House.

Treaties are undemocratic by design. The framers worried that a democratic republic would not be able to present a unified front internationally. But the purpose of treaties was to govern international relations, not submarine domestic legislation under the guise of international relations.

treaties have mostly been, since 1974, a "take it or leave it" fast-track legislation. There's no ability for congress to discuss or modify them -- and there's enormous pressure to pass them -- even if some of the parts are ridiculously unfair, and would be removed in discussion.

In the US, a huge difference between a treaty and a normal law is that only the Senate is involved in ratifying a treaty. The House does not get a vote.

Treaties are one thing, sole-executive agreements are another.