A lot of Google's applications make use of the hamburger menu, including the FABs without descriptive text for the icons.
Why do they still keep at it since most UX researches/analytics have proved that it is bad for user experience?
Unless you are able to get a hold of Google's design team directly, you likely aren't going to get a definitive answer to this beyond the explanations and technical descriptions given in the Material Design documentation for navigation drawers and such. Some of that supporting documentation might shed some light as to be use cases and benefits of using this kind of navigation, and likely are a contributing factor to Google's continued use of the hamburger.
Also, it's worth bearing in mind that there are no ‘good’ or ‘bad’ patterns -- it's all about context! While I would agree that hamburger menus present some navigation discoverability problems and might not be so great when, for example, a user is trying to "find" their way around a website where they have no idea what content is located where (in which case a fully visible main nav would be better)... they are a great way to place content "out of sight, out of mind" for apps whose main focus is a specific task that the user is already aware of and understands (e.g. I'd assume that most users know that the Google Sheets app is for -- surprise! -- editing Google Sheets documents).
Less navigation elements means fewer distractions when users interact with the app. Minimizing the navigation focuses the user’s attention on completing the task.
I'd also like to posit the notion that Google has a great many products in the wild, so their use of the hamburger menu is also likely a means to help standardize the interface between all of their varying products.
Much like the "contextual menu" (the three dots sometimes referred to as a "kebab" menu or "ant" menu), it's become so widespread that it's fairly recognizable by most. In fact, hamburger icons have been used on TV remote controls for quite some time and the glyph even has a unicode character: U+2630. Some designers call for its discontinued use, but that doesn't change the fact that many (most?) users know what it is and how it works by now -- and as such, a great many designers (including those at Google) continue to use it and incorporate it into systems like Material Design. You just have to remember that context is important -- knowing when to use it is key.
I see this question as 2 parts:
I'd like to answer the 1st question.
It has been my experience, researching users for the various projects that I have worked on, that the hamburger menu is NOT bad UX.
In my studies, users habitually (without thinking) go to the hamburger menu when they are looking for very specific things or to accomplish specific tasks.
The problem isn't the Hamburger menu, it's when decisions are made that use 'only' the menu as a point of access.
From what I have seen, there are opportunities for cross-linking between different-yet-related content that should be used to enhance an experience rather than leaning entirely on the nav items (i.e. items hidden in the hamburger menu).
"This page" may contextually have related content on "That page". Being a good UX designer, I will place an access point on "This page" to "That page" because it is part of my users' workflow or thought process - in addition to placing both in the menu. This is 'Contextual placement'.
The problem: However, I have seen some folks strongly argue that 'If its in the nav, we don't need to link to it here.'
The other problem: It is an 'easy out'. Everything under-the-sun being tucked away in the nav to clean up the 'page', rather than digging more deeply into the information design to find a way that is still 'clean' while providing 'usefulness.
The core purpose of the menu: As a mode of navigation for those who know what they are looking for.
Answering the 2nd question:
Why does Google use it? Because the people at Google aim to use it smartly and not as an 'easy out' to proper information design.