As an example, it was often claimed that the US 'signed' and then 'left' the Paris climate treaty, even though it was merely agreed to by the US President, rather than by Congress as required by the US Constitution. Same goes for the Iran nuclear deal. Why is this the case? Shouldn't the international community wait for Congress to concede before counting the US as having 'signed' something?
It is well established in international law that signing a treaty and ratifying a treaty are distinct, contrary to the assumption implicit in the question. See for example the Vienna Convention on the Law of Treaties, which considers throughout that signature and ratification are different, for example in Article 11:
Article 11 — Means of expressing consent to be bound by a treaty
The consent of a State to be bound by a treaty may be expressed by signature, exchange of instruments constituting a treaty, ratification, acceptance, approval or accession, or by any other means if so agreed.
See also Article 14(1)(c) and 14(1)(d):
Article 14 — Consent to be bound by a treaty expressed by ratification, acceptance or approval
The consent of a State to be bound by a treaty is expressed by ratification when:
(c) the representative of the State has signed the treaty subject to ratification; or
(d) the intention of the State to sign the treaty subject to ratification appears from the full powers of its representative or was expressed during the negotiation.
There are several issues here.
First, it is perfectly normal to refer to a treaty as being signed before it is ratified. In fact, a treaty must be signed before it can be ratified.
But based on what you said in the rest of your question, it seems that you may be confusing having signed a treaty, and being a party to a treaty. A country having signed a treaty doesn't mean that it is a party to the treaty -- if the country needs to ratify the treaty, then the country isn't a party to the treaty until it ratifies it. Nevertheless, if the country signed it, it is still correct to say that it signed it, even if the country doesn't ratify it for a long time, or never ratifies it.
Whether a country needs to "ratify" a treaty and how that ratification needs to be done is an internal matter for the country. Different countries have different ratification processes. In the US, some treaties are treated as Article II treaties which are ratified by 2/3 of the Senate. Some treaties are treated as "congressional-executive agreements", which are implemented by regular legislation, passed by a majority of each house of Congress and signed by the President. Some treaties are treated as "executive agreements", which are implemented without any Congressional action.
In the case of the Paris Agreement, the US has signed it, and the US is also a party to it even though it has never "ratified" it. The US decided that the Paris Agreement is an "executive agreement" which can be implemented by using only the powers that the executive branch already has, without needing new legislation from Congress. Therefore, it declared that the ratification was unnecessary and it joined the agreement without ratification. That's why the US is part of the Paris Agreement and would need to "leave" it if it wanted to stop being a party. If, on the other hand, the US had determined that ratification was necessary, and signed it but did not ratify it, then the US would not be a party, and would not need to "leave" it.
Many countries allow the executive of their government to commit the country to a treaty. The US requires ratification by the Senate, but that's an internal matter for the USA, not something that other countries should be involved with. The US gets round this to some degree by allowing many smaller international treaties to be handled as "executive agreements." Those don't usually make the news.
Ratification by the US Senate frequently takes significant time and becomes a political issue within the USA, but that's a consequence of the US form of government. The Australian system, where treaties are normally voted on by the legislature before the executive signs them, has a good deal to recommend it, but requires the legislature to be willing to take action on short notice when requested by the executive. That isn't going to work in the USA.