I personnaly use this google xml sitemap generator http://enarion.net/google/ and it work fine 

there is a cron option

but maybe it will be more easy if it was integrated in elgg

Well enarion script is a bit complex indeed.

I am wondering why elgg's developers pay no attention to SEO - I am looking for a SEO plugin (a possibility to edit a title and a description) and a sitemap plugin - but without success.

Yeah, SEO optimization for elgg should be priority