All so very true. Smaller firms simply don't have the experience/resources/time to prepare "professional" bids for public sector and other big contracts. It's all about dotting the i's, crossing the t's, ensuring the right "words" are in the right places, the right policies in place, etc.
I had the job of doing grant claims for the firm I worked for in the 90s. They were an absolute nightmare, and it was all about matching exactly your application against the published criteria and ensuring your bid matched exactly what the awarding body was expecting to see. That only comes with experience. Once I got a grant claim knocked back by a quango because our application had included the purchase of a particular piece of equipment, let's say the Acme 101x - in reality, the bid took so long for them to approve, the 101x had been superseded by the 101y and we couldn't buy a 101x, so bought the 101y instead which was basically identical, similar price, did the same job, but just a few "tweaks" from the manufacturer. Grant was denied because we didn't "meet the conditions" which was to buy exactly what we had applied for! No amount of persuading, letters from supplier, marketing literature from manufacturer would persuade the awarding body!
Same today with a local theatre trust I work with. The trust spent best part of 20 years trying to get grants to renovate it. Always knocked back. They had a change of management and one of the new trustees was experienced in grant applications, they got awarded the first grant they applied for, for millions! But then, they had trouble finding "acceptable" contractors to do the work, as the bids had to be approved by the awarding body, and all the bids from local contractors, which the trust members knew and trusted, and which were acceptable by them, were rejected by the governing body, for reasons such as not having an adequate anti-bullying policy in one case, another contractor rejected for not declaring the number of disabled employees and gender analysis.
It's nothing to do with whether they can do the work at a suitable price - it's all political and woke these days, and small firms simply don't know how to do all the nit-picking, box-ticking, etc - they are too busy doing the work than to spend all their time doing all the nonsense. The end result is that the "professional" firms get the work and usually do a crap job of it, and probably 2 or 3 times the price, because they've got to employ a small army of people just to write "professional" contract bids containing all the nonsense required.