With respect, I think your premises is incorrect from the outset. “We all know……” 

In fact in the Order, there is no requirement for the ‘assessor’ to be competent as long as he produces a compliant FRA

Your leading phrase perhaps should read, ‘We all know that the Responsible Person has to carry out and record a suitable and sufficient (fire) risk assessment’​​​​​​​

The term ‘competent person’ is mentioned and defined in a couple of areas within the Order - for example in Article 18 - but not in Articles 9 or 11 that refer to risk assessment and emergency procedures.

The rationale is a simple one - the Govt of the day was obsessed with reducing red tape for business and all new legislation had to go through certain tests to ensure it wasnt a burden. 

So - those who guided HMG on fire safety policy argued the following case: - Consider a married couple who own a corner shop with no staff, a meagre turnover and a premises which presents a low risk of fire. Is it reasonable for them to employ an expensive professional for such a risk? They concluded it wasnt.

To back that idea up they mentioned that a SME owner has to provide a huge amount of paperwork to HMRC and others. They usually have the choice of doing themselves or getting someone competent in. 

So fire safety should be manned in a similar way, and to allow DIY FRAs a suite of easy to read (??) Fire Safety Guides were published 

So we have ended up in this curious hybrid of you don't need to be competent, but the document needs to be compliant 

So - from my observations - you get:

The big companies - often with the bigger risk but lots of cash - employing competent fire safety professionals

Smaller one-man-bands with low risk premises may provide a DIY FRA (if you are lucky) but no real harm is done in terms of placing people at risk

Then we have the medium sized SMEs, with medium to high risk premises - maybe single story staircase builds etc. They cant afford a fire constant so ask their H&S guy to have a look. Or they will ask their caretaker to scribble something down. These are where the biggest problems and risks are I have no idea what’s happening in the world of compulsory registering for fire professionals - but post Grenfell - I am surprised its not mandatory - at least for certain types and sizes of buildings

The RRO did not require the fire risk assessor to be competent.  The requirement that the assessment be suitable and sufficient has indeed been taken by some to imply that the assessor must be competent.  You will apparently even find some online summaries of legislation that tell you the assessor has to be competent though this is not stated in the Order.  So the internet knows it, but it's wrong.

Following Grenfell,  the Building Safety Act 2022 has created an amendment to the RRO about assessor competence:

https://www.legislation.gov.uk/ukpga/2022/30/section/156

This introduces a new article 9A, the gist of which is that you must not appoint an assessor who is not competent.

This has however I think not yet been brought into force as the practical definition of competence has not been precisely established (some kind of recognised register of certified assessors is going to be required).  So that's why you don't see it in the current version of the Order.

The reason for the amendment was precisely that the competence of the Grenfell Tower assessor was questioned without a sound legislative basis for competence being a legal requirement.