To a large extent, I think the reason that most of these roles don't go to contractors is one of "trust". When a person is employed as a permie they have made a commitment to the company. They have turned their back on all other opportunities and accepted a role within the organisation. This earns the trust of the company, which in turn allows them to be given high-value, high-impact work - such as architecture roles.

The same can be said of a consultancy. Largely for a consultancy to be considered for high-value, high-impact work, they have to have earned the trust of the customer. This may have been achieved by lots of lower-value, lower-impact work in the past, relationships, or reputation.

It is hard for an individual contractor to gain the sort of trust required in order to be given these roles - some organisations see contracts as "fly-by-night" sorts (the contract market has done itself no favours over the years!) So an answer would seem to be to build a reputation, either via relationships/networks or industry notoriety, or to do a few regular development roles within an organisation in an attempt to gain trust (this of course could backfire, as they may never see you as anything but a developer!)