In many, perhaps all, countries there is the "Constitution" (a legal document) and the "constitution" (the rules and traditions which describe how the government should behave). The constitution includes things like "democratic norms". From time to time parts of the constitution may be written into documents such as "House rules" or even enacted as laws, and while the constitution includes The Constitution, it is wider and more complex.
In the UK, for example, there is no "Constitution" but there is a wide ranging and complex constitution. Many of the Parliamentary systems around the world are modelled, to some extent, on the Westminster system.
In the Westminster system (and in most of the others you describe) the choice of a Speaker is a matter for the house. The Speaker is not chosen by "the people" but by the House from among their members. And the House can remove a Speaker by simple vote. The House is self governing.
So why don't majority parties use their majority to place a speaker who is favorable to them? Well this goes against the constitution (small c) which requires that the Speaker be a fair and neutral representative of the House, and not a partisan. Creating a partisan speaker would be a Pyrrhic victory. One with very few benefits and plenty of problems.
Governments don't need a partisan speaker to enact legislation. They have a majority (perhaps in coalition) so they can set the legislative calendar. They can choose which bills to present and they can get them passed (most of the time)
Governments in democratic countries actually believe in debate! It is known that government bills will get passed, so why have a debate? Because actually having your legislation picked over by someone who disagrees with you is a great way to find problems. It is not unnusual for opposition amendments to be accepted into bills. Now to get effective debate you need a neutral Presiding Officer.
Creating a partisan speaker would by a Pyrrhic victory, because it would weaken the open and democratic system that got the party into power. Over time, systems that are open and democratic have been more resilient than systems in which the party in power consolidates all authority to itself. This is at least partly because the party in power rules better because it must face daily opposition in a fair and neutrally chaired debating chamber. Throw this out and your party will just create more bad laws and be punished in the General Election.
Speakers then have special independence because the Houses that they chair have special independence. Legislative houses are often literally "above the law" in that the judiciary cannot restrict what occurs within the debating chamber. The Speaker represents the House, and while the house is sitting they have "neither eyes to see nor tongue to speak in [the chamber] but as this House is pleased to direct me whose servant [they are]" (as famously spoken by a Speaker in defiance of a King). Speakers have special independence from the King and the Law, but not from the House.
The situation in the USA is rather different, with a government chosen by the President, who is often from the Minority party in the House/Senate. There are different balances in the USA.